Call character checking function with unsigned char

See https://wiki.sei.cmu.edu/confluence/display/c/STR37-C.+Arguments+to+character-handling+functions+must+be+representable+as+an+unsigned+char

AvailableColumnsPanel.c

@@ -45,7 +45,7 @@ static HandlerResult AvailableColumnsPanel_eventHandler(Panel* super, int ch) {
  }
  default:
  {
- if (ch < 255 && isalpha(ch))
+ if (0 < ch && ch < 255 && isalpha((unsigned char)ch))
  result = Panel_selectByTyping(super, ch);
  break;
  }

CategoriesPanel.c

@@ -81,7 +81,7 @@ static HandlerResult CategoriesPanel_eventHandler(Panel* super, int ch) {
  break;
  }
  default:
- if (ch < 255 && isalpha(ch))
+ if (0 < ch && ch < 255 && isalpha((unsigned char)ch))
  result = Panel_selectByTyping(super, ch);
  if (result == BREAK_LOOP)
  result = IGNORED;

ColumnsPanel.c

@@ -91,7 +91,7 @@ static HandlerResult ColumnsPanel_eventHandler(Panel* super, int ch) {
  }
  default:
  {
- if (ch < 255 && isalpha(ch))
+ if (0 < ch && ch < 255 && isalpha((unsigned char)ch))
  result = Panel_selectByTyping(super, ch);
  if (result == BREAK_LOOP)
  result = IGNORED;

IncSet.c

@@ -137,7 +137,7 @@ bool IncSet_handleKey(IncSet* this, int ch, Panel* panel, IncMode_GetPanelValue
  if (size == 0) return true;
  IncMode_find(mode, panel, getPanelValue, 1);
  doSearch = false;
- } else if (ch < 255 && isprint((char)ch)) {
+ } else if (0 < ch && ch < 255 && isprint((unsigned char)ch)) {
  if (mode->index < INCMODE_MAX) {
  mode->buffer[mode->index] = ch;
  mode->index++;

MainPanel.c

@@ -79,7 +79,7 @@ static HandlerResult MainPanel_eventHandler(Panel* super, int ch) {
  } else if (ch != ERR && ch > 0 && ch < KEY_MAX && this->keys[ch]) {
  reaction |= (this->keys[ch])(this->state);
  result = HANDLED;
- } else if (isdigit(ch)) {
+ } else if (0 < ch && ch < 255 && isdigit((unsigned char)ch)) {
  MainPanel_pidSearch(this, ch);
  } else {
  if (ch != ERR) {

Panel.c

@@ -411,7 +411,7 @@ HandlerResult Panel_selectByTyping(Panel* this, int ch) {
  this->eventHandlerState = xCalloc(100, sizeof(char));
  char* buffer = this->eventHandlerState;
 
- if (ch > 0 && ch < 255 && isalnum(ch)) {
+ if (0 < ch && ch < 255 && isalnum((unsigned char)ch)) {
  int len = strlen(buffer);
  if (len < 99) {
  buffer[len] = ch;