Return NULL for invalid chars

This point will never be reached, thanks to throwing an exception, but
a return value is needed to shut the compiler up

src/main/objc/SBJson4StreamWriter.m

@@ -268,6 +268,7 @@ - (BOOL)writeValue:(id)o {
 		case 92: return "\\\\";
 		default:
 			[NSException raise:@"Illegal escape char" format:@"-->%c<-- is not a legal escape character", c];
+			return NULL;
 	}
 }