arg.h: fixed argv checks order

This prevents accessing to a potentially out-of-bounds memory section.

Signed-off-by: Lucas Gabriel Vuotto <l.vuotto92@gmail.com>

arg.h

@@ -10,8 +10,8 @@ extern char *argv0;
 
 /* use main(int argc, char *argv[]) */
 #define ARGBEGIN	for (argv0 = *argv, argv++, argc--;\
-					argv[0] && argv[0][1]\
+					argv[0] && argv[0][0] == '-'\
-					&& argv[0][0] == '-';\
+					&& argv[0][1];\
 					argc--, argv++) {\
 				char argc_;\
 				char **argv_;\