Return ARCHIVE_FATAL if we can't allocate memory instead of going ahead and

dereferencing NULL.

Found by:	Coverity Prevent

lib/libarchive/archive_read_support_format_mtree.c

@@ -596,6 +596,7 @@ read_data(struct archive_read *a, const void **buff, size_t *size, off_t *offset
 			archive_set_error(&a->archive, ENOMEM,
 			    "Can't allocate memory");
 		}
+		return (ARCHIVE_FATAL);
 	}
 
 	*buff = mtree->buff;