Prevent a double free in the error code path

xpmParseDataAndCreate() calls XDestroyImage() in the error path.
Reproducible with sxpm "zero-width.xpm", that file is in the test/
directory.

The same approach is needed in the bytes_per_line == 0 condition though
here it just plugs a memory leak.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>