mmc: core: fix one NULL pointer dereference after SD card is removed

After SD card is removed, the driver would mark its queue DYING to try to
block further more requests from coming into the queue, then clean up its
queue's queuedata by setting it to NULL. However, there can still be new
requests come in right before the DYING mark is set after SD card is
removed. When one new request is allocated and initialized, the queuedata
would be accessed. If queuedata has been cleaned up already, NULL pointer
dereference would happen. This change fixes it by checking if queuedata is
NULL before accessing it, if yes, then bails out with error.

mmc0: card aaaa removed
Buffer I/O error on dev mmcblk0p1, logical block 1, lost async page write
Unable to handle kernel NULL pointer dereference at virtual address
00000000
Mem abort info:
Exception class = DABT (current EL), IL = 32 bits
SET = 0, FnV = 0
EA = 0, S1PTW = 0
Data abort info:
ISV = 0, ISS = 0x00000006
CM = 0, WnR = 0
user pgtable: 4k pages, 39-bit VAs, pgd = ffffffd7bbafa000
[0000000000000000] *pgd=0000000134331003, *pud=0000000134331003,
*pmd=0000000000000000
Internal error: Oops: 96000006 [#1] PREEMPT SMP
task: ffffffd77d193380 task.stack: ffffff8047e30000
pc : mmc_init_request+0x28/0x74
lr : alloc_request_size+0x4c/0x70
...
Process MediaScannerSer (pid: 4710, stack limit = 0xffffff8047e30000)
Call trace:
mmc_init_request+0x28/0x74
alloc_request_size+0x4c/0x70
mempool_alloc+0x104/0x184
get_request+0x324/0x75c
blk_queue_bio+0x154/0x398
generic_make_request+0xcc/0x228
submit_bio+0x13c/0x1d4.

Change-Id: I1967392bd314fa49a723c08873c17bf229d33049
Signed-off-by: Can Guo <cang@codeaurora.org>
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>