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>

drivers/i2c/busses/i2c-bcm2835.c

@@ -195,7 +195,9 @@ static irqreturn_t bcm2835_i2c_isr(int this_irq, void *data)
 	}
 
 	if (val & BCM2835_I2C_S_DONE) {
-		if (i2c_dev->curr_msg->flags & I2C_M_RD) {
+		if (!i2c_dev->curr_msg) {
+			dev_err(i2c_dev->dev, "Got unexpected interrupt (from firmware?)\n");
+		} else if (i2c_dev->curr_msg->flags & I2C_M_RD) {
 			bcm2835_drain_rxfifo(i2c_dev);
 			val = bcm2835_i2c_readl(i2c_dev, BCM2835_I2C_S);
 		}