Fix segfault when involing "lex" without specify the output "-o" and …

…make it work as specified in the "lex.1" man page

main.c

@@ -88,6 +88,7 @@ main(int argc, char **argv)
 	int c;
 	Boolean eoption = 0, woption = 0;
 	const char *driver = 0;
+        const char *fnout = "yy.lex.c";
 
 	sargv = argv;
 	sargc = argc;