staging: r8188eu: add a check for rtw_cbuf_alloc()

kmalloc() is called by rtw_cbuf_alloc() and it returns pointer of
srtuct rtw_cbuf, NULL for allocation failure. So it is better to check
the return value of it.

Signed-off-by: Xiaoke Wang <xkernel.wang@foxmail.com>
Link: https://lore.kernel.org/r/tencent_D95ACC78B93B1CDD14C4E13E4A66FF892008@qq.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>