awk: another smallish code shrink

diff --git a/editors/awk.c b/editors/awk.c
index 44b747b..aac40aa 100644
--- a/editors/awk.c
+++ b/editors/awk.c
@@ -2028,9 +2028,7 @@
 		if (i < 0) i = 0;
 		n = (nargs > 2) ? getvar_i(av[2]) : l-i;
 		if (n < 0) n = 0;
-		s = xzalloc(n + 1);
-		strncpy(s, as[0]+i, n);
-		/*s[n] = '\0'; - xzalloc did it */
+		s = xstrndup(as[0]+i, n);
 		setvar_p(res, s);
 		break;