Heap-use-after free with real-time settings

[derselbst]

While writing tests for #727 I found another heap-use-after free:

fluid_settings_t *settings = new_fluid_settings();
fluid_synth_t * synth = new_fluid_synth();
delete_fluid_synth(synth);

// crashes here, because the real-time setting tries to make a callback to the deleted synth instance
fluid_settings_setint(settings, "synth.polyphony", 2);

[derselbst]

=================================================================
==11608==ERROR: AddressSanitizer: heap-use-after-free on address 0x614000000050 at pc 0x0000004bd630 bp 0x7ffdb9f2cd40 sp 0x7ffdb9f2cd38
READ of size 4 at 0x614000000050 thread T0
    #0 0x4bd62f in fluid_synth_api_enter /fluidsynth/src/synth/fluid_synth.c:6596
    #1 0x4ac754 in fluid_synth_set_polyphony /fluidsynth/src/synth/fluid_synth.c:3231
    #2 0x4ac712 in fluid_synth_handle_polyphony /fluidsynth/src/synth/fluid_synth.c:3215
    #3 0x43d213 in fluid_settings_setint /fluidsynth/src/utils/fluid_settings.c:1559
    #4 0x40f3b5 in main /fluidsynth/test/test_sfont_unloading.c:193
    #5 0x7fbc3ceb5349 in __libc_start_main (/lib64/libc.so.6+0x24349)
    #6 0x40ca79 in _start (/fluidsynth/build/test/test_sfont_unloading+0x40ca79)

0x614000000050 is located 16 bytes inside of 400-byte region [0x614000000040,0x6140000001d0)
freed by thread T0 here:
    #0 0x7fbc4017a1a8 in __interceptor_free (/usr/lib64/libasan.so.4+0xdc1a8)
    #1 0x4402ab in fluid_free /fluidsynth/src/utils/fluid_sys.c:232
    #2 0x4a15d5 in delete_fluid_synth /fluidsynth/src/synth/fluid_synth.c:1145
    #3 0x40f07a in test_default_polyphony /fluidsynth/test/test_sfont_unloading.c:179
    #4 0x40f387 in main /fluidsynth/test/test_sfont_unloading.c:190
    #5 0x7fbc3ceb5349 in __libc_start_main (/lib64/libc.so.6+0x24349)

previously allocated by thread T0 here:
    #0 0x7fbc4017a500 in malloc (/usr/lib64/libasan.so.4+0xdc500)
    #1 0x44025a in fluid_alloc /fluidsynth/src/utils/fluid_sys.c:203
    #2 0x49cd93 in new_fluid_synth /fluidsynth/src/synth/fluid_synth.c:622
    #3 0x40dbd3 in test_default_polyphony /fluidsynth/test/test_sfont_unloading.c:113
    #4 0x40f387 in main /fluidsynth/test/test_sfont_unloading.c:190
    #5 0x7fbc3ceb5349 in __libc_start_main (/lib64/libc.so.6+0x24349)

SUMMARY: AddressSanitizer: heap-use-after-free /fluidsynth/src/synth/fluid_synth.c:6596 in fluid_synth_api_enter
Shadow bytes around the buggy address:
  0x0c287fff7fb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c287fff7fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c287fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c287fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c287fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c287fff8000: fa fa fa fa fa fa fa fa fd fd[fd]fd fd fd fd fd
  0x0c287fff8010: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c287fff8020: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c287fff8030: fd fd fd fd fd fd fd fd fd fd fa fa fa fa fa fa
  0x0c287fff8040: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c287fff8050: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==11608==ABORTING