ExprPixelFunc(): fix memory leak in error code path

[rouault]

Fixes https://github.com/OSGeo/gdal/actions/runs/12718613957/job/35457381239?pr=11209

2025-01-11T01:23:20.3982957Z =================================================================
2025-01-11T01:23:20.3983380Z ==11530==ERROR: LeakSanitizer: detected memory leaks
2025-01-11T01:23:20.3983629Z 
2025-01-11T01:23:20.3987049Z Direct leak of 8 byte(s) in 1 object(s) allocated from:
2025-01-11T01:23:20.3996994Z     #0 0x7fdd70b2f7a3 in malloc (/usr/lib/llvm-18/lib/clang/18/lib/linux/libclang_rt.asan-x86_64.so+0xf87a3) (BuildId: bf9e3747052c46cd49f7974555b1700dce30b8c0)
2025-01-11T01:23:20.3998358Z     #1 0x7fdd5acc7e3c in VSIMalloc /home/runner/work/gdal/gdal/port/cpl_vsisimple.cpp:555:12
2025-01-11T01:23:20.3999219Z     #2 0x7fdd5ac054f6 in CPLMalloc /home/runner/work/gdal/gdal/port/cpl_conv.cpp:193:21
2025-01-11T01:23:20.4000506Z     #3 0x7fdd5d569c6b in ExprPixelFunc(void**, int, void*, int, int, GDALDataType, GDALDataType, int, int, char const* const*) /home/runner/work/gdal/gdal/frmts/vrt/pixelfunctions.cpp:1685:31
2025-01-11T01:23:20.4003893Z     #4 0x7fdd5d3a8a20 in CPLErr std::__invoke_impl<CPLErr, CPLErr (*&)(void**, int, void*, int, int, GDALDataType, GDALDataType, int, int, char const* const*), void**, int, void*, int, int, GDALDataType, GDALDataType, int, int, char const* const*>(std::__invoke_other, CPLErr (*&)(void**, int, void*, int, int, GDALDataType, GDALDataType, int, int, char const* const*), void**&&, int&&, void*&&, int&&, int&&, GDALDataType&&, GDALDataType&&, int&&, int&&, char const* const*&&) /usr/bin/../lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/invoke.h:61:14
2025-01-11T01:23:20.4009486Z     #5 0x7fdd5d3a830a in std::enable_if<is_invocable_r_v<CPLErr, CPLErr (*&)(void**, int, void*, int, int, GDALDataType, GDALDataType, int, int, char const* const*), void**, int, void*, int, int, GDALDataType, GDALDataType, int, int, char const* const*>, CPLErr>::type std::__invoke_r<CPLErr, CPLErr (*&)(void**, int, void*, int, int, GDALDataType, GDALDataType, int, int, char const* const*), void**, int, void*, int, int, GDALDataType, GDALDataType, int, int, char const* const*>(CPLErr (*&)(void**, int, void*, int, int, GDALDataType, GDALDataType, int, int, char const* const*), void**&&, int&&, void*&&, int&&, int&&, GDALDataType&&, GDALDataType&&, int&&, int&&, char const* const*&&) /usr/bin/../lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/invoke.h:114:9
2025-01-11T01:23:20.4014368Z     #6 0x7fdd5d3a7c76 in std::_Function_handler<CPLErr (void**, int, void*, int, int, GDALDataType, GDALDataType, int, int, char const* const*), CPLErr (*)(void**, int, void*, int, int, GDALDataType, GDALDataType, int, int, char const* const*)>::_M_invoke(std::_Any_data const&, void**&&, int&&, void*&&, int&&, int&&, GDALDataType&&, GDALDataType&&, int&&, int&&, char const* const*&&) /usr/bin/../lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/std_function.h:290:9
2025-01-11T01:23:20.4017879Z     #7 0x7fdd5d3a5c9f in std::function<CPLErr (void**, int, void*, int, int, GDALDataType, GDALDataType, int, int, char const* const*)>::operator()(void**, int, void*, int, int, GDALDataType, GDALDataType, int, int, char const* const*) const /usr/bin/../lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/std_function.h:591:9
2025-01-11T01:23:20.4020358Z     #8 0x7fdd5d39a7be in VRTDerivedRasterBand::IRasterIO(GDALRWFlag, int, int, int, int, void*, int, int, GDALDataType, long long, long long, GDALRasterIOExtraArg*) /home/runner/work/gdal/gdal/frmts/vrt/vrtderivedrasterband.cpp:1392:16
2025-01-11T01:23:20.4022581Z     #9 0x7fdd630a0310 in GDALRasterBand::RasterIO(GDALRWFlag, int, int, int, int, void*, int, int, GDALDataType, long long, long long, GDALRasterIOExtraArg*) /home/runner/work/gdal/gdal/gcore/gdalrasterband.cpp:444:13
2025-01-11T01:23:20.4024168Z     #10 0x7fdd630a19ee in GDALRasterIOEx /home/runner/work/gdal/gdal/gcore/gdalrasterband.cpp:505:21
2025-01-11T01:23:20.4026217Z     #11 0x7fdd4a4ca40a in BandRasterIONumPy(void*, int, double, double, double, double, tagPyArrayObject*, GDALDataType, GDALRIOResampleAlg, int (*)(double, char const*, void*), void*) /home/runner/work/gdal/gdal/build-asan/swig/python/extensions/gdal_array_wrap.cpp:4769:27

[dbaston]

applied locally