Free up debug strings when deallocating game-data

This fixes a memory leak, which also occurs in the original game

code/fgame/g_main.cpp

@@ -227,9 +227,13 @@ void G_AllocGameData(void)
 
 void G_DeAllocGameData(void)
 {
-    // Initialize debug lines
+    // Free up debug lines
     G_DeAllocDebugLines();
 
+    // Added in OPM
+    //  Free up debug strings
+    G_DeAllocDebugStrings();
+
     // free up the entities
     if (g_entities) {
         gi.Free(g_entities);