Fix Memory Leak: pngwriter::readfromfile

... did not free the png_ptr and info_ptr objects again.
If getting the info_ptr fails this might still leak but the
cases "success" and "file-not-found" are fixed now.

Related to #5, might fix
http://sourceforge.net/p/pngwriter/discussion/238247/thread/15ee786c/

Thanks to @psychocoderHPC for pushing me into valgrind :)