chg: fix an undefined behavior about memcpy

Spot by Wez Furlong.  `memcpy(x, NULL, 0)` is undefined according to [1].

[1]: https://stackoverflow.com/questions/5243012

Differential Revision: https://phab.mercurial-scm.org/D3698

contrib/chg/chg.c

@@ -220,7 +220,10 b' static void execcmdserver(const struct c'
 
 	const char **argv = mallocx(sizeof(char *) * argsize);
 	memcpy(argv, baseargv, sizeof(baseargv));
-	memcpy(argv + baseargvsize, opts->args, sizeof(char *) * opts->argsize);
+	if (opts->args) {
+		size_t size = sizeof(char *) * opts->argsize;
+		memcpy(argv + baseargvsize, opts->args, size);
+	}
 	argv[argsize - 1] = NULL;
 
 	if (putenv("CHGINTERNALMARK=") != 0)