[sfnt] Fix memory leak in png loading
Reported as
https://bugs.chromium.org/p/chromium/issues/detail?id=1182552
Memory is allocated and the pointer assigned to 'rows' inside a setjmp scope. This memory must be freed outside the setjmp scope after a longjmp. Since 'rows' is a local and modified inside the setjmp scope it must be marked volatile or it will have indeterminite value after the longjmp.
- src/sfnt/pngshim.c (Load_SBit_Png): Fix memory leak of 'rows'.