Fix CVE-2023-43788: Out of bounds read in XpmCreateXpmImageFromBuffer

When the test case for CVE-2022-46285 was run with the Address Sanitizer
enabled, it found an out-of-bounds read in ParseComment() when reading
from a memory buffer instead of a file, as it continued to look for the
closing comment marker past the end of the buffer.

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