delta-apply.c: Use GIT_ENOMEM instead of GIT_ERROR when allocating

git__delta_apply used to return with a generic GIT_ERROR when allocating
memory for the delta failed.

Fix this to return GIT_ENOMEM.

src/delta-apply.c

@@ -52,7 +52,7 @@ int git__delta_apply(
 		return GIT_ERROR;
 
 	if ((res_dp = git__malloc(res_sz + 1)) == NULL)
-		return GIT_ERROR;
+		return GIT_ENOMEM;
 	res_dp[res_sz] = '\0';
 	out->data = res_dp;
 	out->len = res_sz;