mymalloc(): don't cast return value; void* is always promoted
mymalloc sets allocated memory to zero; no need for extra memset
netcam.c: tfile_context(): simplify function

This function always returned `ret`, so might as well take a shortcut
and return the pointer directly.
free(): remove unnecessary NULL pointer checks

Calling free() on a NULL pointer is a completely harmless. Reduce code
complexity by removing the unnecessary checks.
Merge pull request #50 from aklomp/mymalloc

Tweaks to uses of 'mymalloc'