drivers/md/md-bitmap: check the return value of md_bitmap_get_counter()

Check the return value of md_bitmap_get_counter() in case it returns
NULL pointer, which will result in a null pointer dereference.

v2: update the check to include other dereference
Signed-off-by: Li Zhong <floridsleeves@gmail.com>
Signed-off-by: Song Liu <song@kernel.org>

drivers/md/md-bitmap.c

@@ -2195,10 +2195,12 @@ int md_bitmap_resize(struct bitmap *bitmap, sector_t blocks,
 
 		if (set) {
 			bmc_new = md_bitmap_get_counter(&bitmap->counts, block, &new_blocks, 1);
+			if (bmc_new) {
 				if (*bmc_new == 0) {
 					/* need to set on-disk bits too. */
 					sector_t end = block + new_blocks;
 					sector_t start = block >> chunkshift;
+
 					start <<= chunkshift;
 					while (start < end) {
 						md_bitmap_file_set_bit(bitmap, block);
@@ -2209,6 +2211,7 @@ int md_bitmap_resize(struct bitmap *bitmap, sector_t blocks,
 					md_bitmap_set_pending(&bitmap->counts, block);
 				}
 				*bmc_new |= NEEDED_MASK;
+			}
 			if (new_blocks < old_blocks)
 				old_blocks = new_blocks;
 		}