runat.c: Add an explicit check for snprintf() failure

The check for "outsiz" too large was probably sufficient to
catch failures, since it was cast to an unsigned (size_t).
However, it seems appropriate to add an explicit check for
a failed case (returning -1).

Discussed with:	oshogbo
Fixes:	0660de8172 ("runat: Add a runat(1) utility similar to the Solaris one")

usr.bin/runat/runat.c

@@ -52,6 +52,8 @@ main(int argc, char *argv[])
 	pos = 0;
 	for (i = 1; i < argc; i++) {
 		outsiz = snprintf(&buf[pos], siz, "%s ", argv[i]);
+		if (outsiz <= 0)
+			errx(1, "snprintf failed: returned %d", outsiz);
 		if ((size_t)outsiz > siz)
 			errx(1, "Arguments too large");
 		pos += outsiz;