Handle unexpected values for character passed to isalnum

It seems that certain negative integer values can crash isalnum().
Let's protect against those.

Fixes #711.

Panel.c

@@ -469,7 +469,7 @@ HandlerResult Panel_selectByTyping(Panel* this, int ch) {
       this->eventHandlerState = xCalloc(100, sizeof(char));
    char* buffer = this->eventHandlerState;
 
-   if (ch < 255 && isalnum(ch)) {
+   if (ch > 0 && ch < 255 && isalnum(ch)) {
       int len = strlen(buffer);
       if (len < 99) {
          buffer[len] = ch;