Fix null pointer dereference
Fix null pointer dereference

write_string_to_file would try to write the error message that it failed to open a file stream to the very same file stream it just tried to open.
Fix this by writing to stderr instead.
KhronosGroup/SPIRV-Cross,h3xl3r/SPIRV-Cross,KhronosGroup/SPIRV-Cross,h3xl3r/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,h3xl3r/SPIRV-Cross,KhronosGroup/SPIRV-Cross,h3xl3r/SPIRV-Cross,KhronosGroup/SPIRV-Cross,h3xl3r/SPIRV-Cross