cc: dont' try compiling invalid code

When we have errors in the code, passing it on to the backend
just generates cryptic or incorrect errors, and in some case,
crashes as we try to process trees with missing labels, or
which are malformed in other ways.

Instead, bail out of codgen() and don't even try.

sys/src/cmd/cc/pgen.c

@@ -11,6 +11,11 @@ codgen(Node *n, Node *nn)
 	maxargsafe = 0;
 	hasdoubled = 0;
 
+	/*
+	 * no point in giving invalid code to the backend.
+	 */
+	if(nerrors != 0)
+		return;
 	/*
 	 * isolate name
 	 */