Import arg.h OOB fix from Lucas Gabriel Vuotto.

This is a small fix for arg.h where it's possible to perform an out-of-boundary
memory access in argv's internal string.

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_;\