xdiff: handle allocation failure when merging

Other users of xdiff such as libgit2 need to be able to handle
allocation failures. These allocation failures were previously
ignored.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

xdiff/xmerge.c

@@ -708,13 +708,18 @@ int xdl_merge(mmfile_t *orig, mmfile_t *mf1, mmfile_t *mf2,
 	    xdl_build_script(&xe2, &xscr2) < 0)
 		goto out;
 
-	status = 0;
 	if (!xscr1) {
 		result->ptr = xdl_malloc(mf2->size);
+		if (!result->ptr)
+			goto out;
+		status = 0;
 		memcpy(result->ptr, mf2->ptr, mf2->size);
 		result->size = mf2->size;
 	} else if (!xscr2) {
 		result->ptr = xdl_malloc(mf1->size);
+		if (!result->ptr)
+			goto out;
+		status = 0;
 		memcpy(result->ptr, mf1->ptr, mf1->size);
 		result->size = mf1->size;
 	} else {