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: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/core/sdio_io.c

@@ -373,6 +373,7 @@ u8 sdio_readb(struct sdio_func *func, unsigned int addr, int *err_ret)
 	u8 val;
 
 	if (!func) {
+		if (err_ret)
 			*err_ret = -EINVAL;
 		return 0xFF;
 	}
@@ -407,6 +408,7 @@ void sdio_writeb(struct sdio_func *func, u8 b, unsigned int addr, int *err_ret)
 	int ret;
 
 	if (!func) {
+		if (err_ret)
 			*err_ret = -EINVAL;
 		return;
 	}
@@ -635,6 +637,7 @@ unsigned char sdio_f0_readb(struct sdio_func *func, unsigned int addr,
 	unsigned char val;
 
 	if (!func) {
+		if (err_ret)
 			*err_ret = -EINVAL;
 		return 0xFF;
 	}
@@ -673,6 +676,7 @@ void sdio_f0_writeb(struct sdio_func *func, unsigned char b, unsigned int addr,
 	int ret;
 
 	if (!func) {
+		if (err_ret)
 			*err_ret = -EINVAL;
 		return;
 	}