awk: errror on printf format strings lacking conversion specifier

Reported by:	phk
Reviewed by:	imp, markj
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D39573

contrib/one-true-awk/run.c

@@ -912,6 +912,9 @@ int format(char **pbuf, int *pbufsize, const char *s, Node *a)	/* printf-like co
 		case 'c':
 			flag = 'c';
 			break;
+		case '\0':
+			FATAL("missing printf conversion specifier");
+			break;
 		default:
 			WARNING("weird printf conversion %s", fmt);
 			flag = '?';