memory corruption loading files from pngsuite

[3b]

http://www.schaik.com/pngsuite/tbbn0g04.png gets an alpha channel due to (png-set-expand png-ptr), but only 1 channel is allocated in the make-image call, so png-read-image writes past the end of the array.

Also, depending on options and file, both png-set-expand and png-set-strip-alpha are called, and the docs say

If you specify a contradictory set of transformations, for example both adding and removing the alpha channel, you cannot predict the final result.