Make sure start pointer is legal in strtaglen()

strtaglen() takes care not go fall off the edge off the world... except
if you make it start there. Oops.

lib/header.c

@@ -351,6 +351,8 @@ static inline int strtaglen(const char *str, rpm_count_t c, const char *end)
     const char *s;
 
     if (end) {
+	if (str >= end)
+	    return -1;
 	while ((s = memchr(start, '\0', end-start))) {
 	    if (--c == 0 || s > end)
 		break;