remoteproc: Check for NULL firmwares in sysfs interface

The remoteproc framework provides a sysfs file 'firmware'
for modifying the firmware image name from userspace. Add
an additional check to ensure NULL firmwares are errored
out right away, rather than getting a delayed error while
requesting a firmware during the start of a remoteproc
later on.
Tested-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

drivers/remoteproc/remoteproc_sysfs.c

@@ -48,6 +48,11 @@ static ssize_t firmware_store(struct device *dev,
 	}
 
 	len = strcspn(buf, "\n");
+	if (!len) {
+		dev_err(dev, "can't provide a NULL firmware\n");
+		err = -EINVAL;
+		goto out;
+	}
 
 	p = kstrndup(buf, len, GFP_KERNEL);
 	if (!p) {