mmc: mmc_test: Fix possible NULL pointer dereference

test->highmem can be NULL if no page can be allocated.
In this case don't try to free it which can cause NULL
pointer dereference crash.

Change-Id: I6c31a265fd801dfc4169530e3f39ffcf6590f5a8
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>
diff --git a/drivers/mmc/card/mmc_test.c b/drivers/mmc/card/mmc_test.c
index a2b005e..9063e98 100644
--- a/drivers/mmc/card/mmc_test.c
+++ b/drivers/mmc/card/mmc_test.c
@@ -2889,7 +2889,8 @@
 	}
 
 #ifdef CONFIG_HIGHMEM
-	__free_pages(test->highmem, BUFFER_ORDER);
+	if (test->highmem)
+		__free_pages(test->highmem, BUFFER_ORDER);
 #endif
 	kfree(test->buffer);
 	kfree(test);