staging: r8188eu: Fix potential memory leak or NULL dereference

staging: r8188eu: Fix potential memory leak or NULL dereference

In routine c2h_wk_callback(() following a kmalloc() call, the
error recovery is flawed. If the kmalloc() returns a pointer
that is not NULL, and the following c2h_evt_read() fails, the
code will leak that buffer. If the kmalloc() fails, a NULL
dereference will occur in the following code.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Link: https://lore.kernel.org/r/20210806213235.22349-1-Larry.Finger@lwfinger.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>