drivers: i2c_ll_stm32_v2: Test slave_cfg pointer before using it

Since the introduction of the second slave address support, slave_cfg
pointer may be NULL when entering slave event function.
It happens when both targets are registered and then the first one is
unregsitered.

Test the pointer vs NULL before using it.

Signed-off-by: Vincent Geneves <vgeneves@kalray.eu>