staging: r8188eu: add check for kzalloc

staging: r8188eu: add check for kzalloc

[ Upstream commit f94b47c6bde624d6c07f43054087607c52054a95 ]

As kzalloc() may return null pointer, it should be better to
check the return value and return error if fails in order
to avoid dereference of null pointer.
Moreover, the return value of rtw_alloc_hwxmits() should also
be dealt with.

Fixes: 15865124feed ("staging: r8188eu: introduce new core dir for RTL8188eu driver")
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Link: https://lore.kernel.org/r/20220518075957.514603-1-jiasheng@iscas.ac.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>