spi: Always check complete callback before calling it

Since commit 1e25cd4729bd "spi: Do not require a completion", this checking is
required to prevent NULL pointer dereference.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
diff --git a/drivers/spi/spi-sh.c b/drivers/spi/spi-sh.c
index f6f2c70..03edf5e 100644
--- a/drivers/spi/spi-sh.c
+++ b/drivers/spi/spi-sh.c
@@ -322,7 +322,8 @@
 		spin_lock_irqsave(&ss->lock, flags);
 
 		mesg->status = 0;
-		mesg->complete(mesg->context);
+		if (mesg->complete)
+			mesg->complete(mesg->context);
 	}
 
 	clear_fifo(ss);
@@ -340,7 +341,8 @@
 
  error:
 	mesg->status = ret;
-	mesg->complete(mesg->context);
+	if (mesg->complete)
+		mesg->complete(mesg->context);
 
 	spi_sh_clear_bit(ss, SPI_SH_SSA | SPI_SH_SSDB | SPI_SH_SSD,
 			 SPI_SH_CR1);