xdiff: catch memory allocation errors

The xdiff code contains multiple call sites where the results of
`xdl_malloc` are not being checked for memory allocation errors.

Add checks to fix possible segfaults due to `NULL` pointer accesses.

src/xdiff/xmerge.c

@@ -717,10 +717,22 @@ int xdl_merge(mmfile_t *orig, mmfile_t *mf1, mmfile_t *mf2,
 	status = 0;
 	if (!xscr1) {
 		result->ptr = xdl_malloc(mf2->size);
+		if (!result->ptr) {
+			xdl_free_script(xscr2);
+			xdl_free_env(&xe1);
+			xdl_free_env(&xe2);
+			return -1;
+		}
 		memcpy(result->ptr, mf2->ptr, mf2->size);
 		result->size = mf2->size;
 	} else if (!xscr2) {
 		result->ptr = xdl_malloc(mf1->size);
+		if (!result->ptr) {
+			xdl_free_script(xscr1);
+			xdl_free_env(&xe1);
+			xdl_free_env(&xe2);
+			return -1;
+		}
 		memcpy(result->ptr, mf1->ptr, mf1->size);
 		result->size = mf1->size;
 	} else {

src/xdiff/xpatience.c

@@ -217,6 +217,9 @@ static struct entry *find_longest_common_sequence(struct hashmap *map)
 	 */
 	int anchor_i = -1;
 
+	if (!sequence)
+		return NULL;
+
 	for (entry = map->first; entry; entry = entry->next) {
 		if (!entry->line2 || entry->line2 == NON_UNIQUE)
 			continue;