Workaround double free problems

The source code for handling images has a double free problem. Recent versions
of the GNU libc have a mechanism to automagically abort the program in such a
case. This causes Mosaic to crash if images are displayed.

I work around this issue by turning of automatical program abortion with a call
to mallopt().
Enable compilation with clang

Some functions that ought to return a value call return without any value. This
commit adds a dummy value to each of those returns so that clang stops
complaining.