i2c: bcm2835: Avoid possible NULL ptr dereference

Since commit e2474541 ("bcm2835: Fix hang for writing messages
larger than 16 bytes") the interrupt handler is prone to a possible
NULL pointer dereference. This could happen if an interrupt fires
before curr_msg is set by bcm2835_i2c_xfer_msg() and randomly occurs
on the RPi 3. Even this is an unexpected behavior the driver must
handle that with an error instead of a crash.

Reported-by: Peter Robinson <pbrobinson@gmail.com>
Fixes: e2474541 ("bcm2835: Fix hang for writing messages larger than 16 bytes")
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>