contrib: Apply various fixes to libtests

pngimage.c:
Initialize sig_bits on a NOTREACHED path to avoid warnings about using
uninitialized variables.

pngstest.c:
Enlarge buffers and fix signedness to avoid legitimate warnings about
potential buffer overflows.

pngunknown.c:
pngvalid.c:
Use NULL instead of 0 for pointers and apply other style fixes.

makepng.c:
tarith.c:
Apply various style fixes.

Also remove the "last changed" version info from source comments.
The version control system maintains this information automatically.
9 files changed