Don't crash if "gps" is null

library/modules/Gui.cpp

@@ -998,16 +998,29 @@ bool Gui::setDesignationCoords (const int32_t x, const int32_t y, const int32_t
 
 bool Gui::getMousePos (int32_t & x, int32_t & y)
 {
+    if (gps) {
         x = gps->mouse_x;
         y = gps->mouse_y;
+    }
+    else {
+        x = -1;
+        y = -1;
+    }
     return (x == -1) ? false : true;
 }
 
 bool Gui::getWindowSize (int32_t &width, int32_t &height)
 {
+    if (gps) {
         width = gps->dimx;
         height = gps->dimy;
         return true;
+    }
+    else {
+        width = 80;
+        height = 25;
+        return false;
+    }
 }
 
 bool Gui::getMenuWidth(uint8_t &menu_width, uint8_t &area_map_width)

library/modules/Windows.cpp

@@ -34,10 +34,12 @@ distribution.
 #include "modules/Windows.h"
 
 using namespace DFHack;
+using df::global::gps;
 
 Windows::df_screentile *Windows::getScreenBuffer()
 {
-    return (df_screentile *) df::global::gps->screen;
+    if (!gps) return NULL;
+    return (df_screentile *) gps->screen;
 }
 
 Windows::df_window::df_window(int x, int y, unsigned int width, unsigned int height)
@@ -79,7 +81,7 @@ bool Windows::df_window::unlock (painter * painter)
     return false;
 }
 
-Windows::top_level_window::top_level_window(): df_window(0,0,df::global::gps->dimx,df::global::gps->dimy)
+Windows::top_level_window::top_level_window() : df_window(0,0,gps ? gps->dimx : 80,gps ? gps->dimy : 25)
 {
     buffer = 0;
 }