arch/sim: Fix X11 segmentation fault when CONFIG_SIM_X11NOSHM isn't enabled

up_x11mapsharedmem should assign the share memory to g_framebuffer

arch/sim/src/sim/up_x11framebuffer.c

@@ -317,8 +317,8 @@ static inline int up_x11mapsharedmem(int depth, unsigned int fblen)
           goto shmerror;
         }
 
+      g_framebuffer = (unsigned char *)g_image->data;
       g_shmcheckpoint++;
-
     }
   else
 #endif