cmd/5a, cmd/6a, cmd/8a: fix flag parsing

go tool 6a -$(unicode fffd) was crashing.

Fixes #5878.

R=ken2
CC=golang-dev
https://golang.org/cl/11208045

src/cmd/5a/lex.c

@@ -68,7 +68,7 @@ main(int argc, char *argv[])
 	ARGBEGIN {
 	default:
 		c = ARGC();
-		if(c >= 0 || c < sizeof(debug))
+		if(c >= 0 && c < sizeof(debug))
 			debug[c] = 1;
 		break;
 

src/cmd/6a/lex.c

@@ -74,7 +74,7 @@ main(int argc, char *argv[])
 	ARGBEGIN {
 	default:
 		c = ARGC();
-		if(c >= 0 || c < sizeof(debug))
+		if(c >= 0 && c < sizeof(debug))
 			debug[c] = 1;
 		break;
 

src/cmd/8a/lex.c

@@ -74,7 +74,7 @@ main(int argc, char *argv[])
 	ARGBEGIN {
 	default:
 		c = ARGC();
-		if(c >= 0 || c < sizeof(debug))
+		if(c >= 0 && c < sizeof(debug))
 			debug[c] = 1;
 		break;