Fix compiler warnings: clobber variables

This patchs fixes warnings generated by older toolchains:

shared/image-loader.c: In function 'load_png':
shared/image-loader.c:211:12: warning: variable 'data' might be clobbered by 'longjmp' or 'vfork' [-Wclobbered]
  png_byte *data = NULL;

See https://linux.die.net/man/3/longjmp

 why is this needed.

Signed-off-by: Marius Vlad <marius.vlad0@gmail.com>