mmc: core: guard dereference of optional parameter

Various functions take as parameter an optional pointer. Pointer
should be guarded with non-NULL check before dereferencing.

Add non-NULL check before dereference of pointer.
Signed-off-by: NTobin C. Harding <me@tobin.cc>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>

drivers/mmc/core/sdio_io.c

@@ -373,7 +373,8 @@ u8 sdio_readb(struct sdio_func *func, unsigned int addr, int *err_ret)
 	u8 val;
 
 	if (!func) {
-		*err_ret = -EINVAL;
+		if (err_ret)
+			*err_ret = -EINVAL;
 		return 0xFF;
 	}
 
@@ -407,7 +408,8 @@ void sdio_writeb(struct sdio_func *func, u8 b, unsigned int addr, int *err_ret)
 	int ret;
 
 	if (!func) {
-		*err_ret = -EINVAL;
+		if (err_ret)
+			*err_ret = -EINVAL;
 		return;
 	}
 
@@ -635,7 +637,8 @@ unsigned char sdio_f0_readb(struct sdio_func *func, unsigned int addr,
 	unsigned char val;
 
 	if (!func) {
-		*err_ret = -EINVAL;
+		if (err_ret)
+			*err_ret = -EINVAL;
 		return 0xFF;
 	}
 
@@ -673,7 +676,8 @@ void sdio_f0_writeb(struct sdio_func *func, unsigned char b, unsigned int addr,
 	int ret;
 
 	if (!func) {
-		*err_ret = -EINVAL;
+		if (err_ret)
+			*err_ret = -EINVAL;
 		return;
 	}