Need to restore STDERR in pngtest.c

[bungeman]

In test: Add consistency checks for the PNG_LIBPNG_VER* number the STDERR macro was moved from outside an #ifdef to inside an #ifdef. This broke the code in the else of this #ifdef which also uses the STDERR macro. It appears the #define STDERR should be moved back to where it was to avoid compile errors in the #else case.

This can be reproduced by making a change similar to what the oss-fuzz build currently does setting option WRITE disabled and then building.