ccmake: Free main form before destroying cache entry fields

InitializeUI() replaced the cache entry widgets and the "EMPTY CACHE"
placeholder, and RemoveEntry() erased an entry, while the form still
referenced their fields.  free_field() fails on connected fields, so
every reconfigure and every deleted entry leaked fields.  Free the form
first; RePost() and Render() create a new one afterwards.

Source/CursesDialog/cmCursesMainForm.cxx

@@ -74,6 +74,13 @@ void cmCursesMainForm::InitializeUI()
   // Hide the cursor by default
   curs_set(0);
 
+  // Free the form before destroying the fields it references
+  if (this->Form) {
+    unpost_form(this->Form);
+    free_form(this->Form);
+    this->Form = nullptr;
+  }
+
   // Create a vector of cmCursesCacheEntryComposite's
   // which contain labels, entries and new entry markers
   std::vector<cmCursesCacheEntryComposite> newEntries;
@@ -620,6 +627,12 @@ void cmCursesMainForm::RemoveEntry(char const* value)
 
   if (removeIt != this->Entries.end()) {
     this->CMakeInstance->UnwatchUnusedCli(value);
+    // Free the form before destroying the fields it references
+    if (this->Form) {
+      unpost_form(this->Form);
+      free_form(this->Form);
+      this->Form = nullptr;
+    }
     this->Entries.erase(removeIt);
   }
 }