krnl386.exe: Don't try to free again an already freed global block.

dlls/krnl386.exe16/global.c

@@ -182,6 +182,11 @@ BOOL16 GLOBAL_FreeBlock( HGLOBAL16 handle )
     sel = GlobalHandleToSel16( handle );
     if (!VALID_HANDLE(sel)) return FALSE;
     pArena = GET_ARENA_PTR(sel);
+    if (!pArena->size)
+    {
+        WARN( "already free %x\n", handle );
+        return FALSE;
+    }
     SELECTOR_FreeBlock( sel );
     memset( pArena, 0, sizeof(GLOBALARENA) );
     return TRUE;