SIGSEGV if nativejson is run from certain directories

[cebtenzzre]

If the nativejson binary is run from certain directories (such as the root of the repository), ReadFiles fails twice:

nativejson-benchmark/src/main.cpp

Line 1238 in 2d4e633

if (!ReadFiles("../data/%s", testJsons))

So testJsons has length 0 and capacity 0. That makes testJsons.front() invalid, as no memory has been allocated for the vector yet. So here:

nativejson-benchmark/src/main.cpp

Line 726 in 2d4e633

char * const argv[] = { fullpath, testJsons.front().fullpath, NULL };

A null pointer is dereferenced, causing a segfault.