wing32: Avoid crash in WinGGetDIBPointer when called with NULL bitmap info.

dlls/wing32/tests/wing32.c

@@ -51,6 +51,9 @@ static void test_WinGGetDIBPointer(void)
     dib = WinGGetDIBPointer(NULL, NULL);
     ok(dib == NULL, "WinGGetDIBPointer returned unexpected value %p\n", dib);
 
+    dib = WinGGetDIBPointer(bmp, NULL);
+    ok(dib != NULL, "WinGGetDIBPointer failed\n");
+
     dib = WinGGetDIBPointer(bmp, &bmi);
     ok(dib != NULL, "WinGGetDIBPointer failed\n");
 

dlls/wing32/wing32.c

@@ -72,7 +72,7 @@ void * WINAPI WinGGetDIBPointer( HBITMAP hbmp, BITMAPINFO *bmi )
 
     if (GetObjectW( hbmp, sizeof(ds), &ds ) == sizeof(ds))
     {
-        bmi->bmiHeader = ds.dsBmih;
+        if (bmi) bmi->bmiHeader = ds.dsBmih;
         return ds.dsBm.bmBits;
     }
     return NULL;