staging: rtl8712: Fix a possible NULL pointer dereference in function r8712_joinbss_event_callback

GCC 10 analyzer reports a warning: dereference of NULL
The function r8712_find_network can return NULL and is usually checked but
no check is present is this case.
Fix by adding the check.

Signed-off-by: Lee Gibson <leegib@gmail.com>
Link: https://lore.kernel.org/r/20210319085836.8259-1-leegib@gmail.com


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