arm64: kexec_file: handle empty command-line

Calling strlen() on cmdline == NULL produces a kernel oops. Since having
a NULL cmdline is valid, handle this case explicitly.

Fixes: 52b2a8af ("arm64: kexec_file: load initrd and device-tree")
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/machine_kexec_file.c

@@ -120,10 +120,12 @@ static int create_dtb(struct kimage *image,
 {
 	void *buf;
 	size_t buf_size;
+	size_t cmdline_len;
 	int ret;
 
+	cmdline_len = cmdline ? strlen(cmdline) : 0;
 	buf_size = fdt_totalsize(initial_boot_params)
-			+ strlen(cmdline) + DTB_EXTRA_SPACE;
+			+ cmdline_len + DTB_EXTRA_SPACE;
 
 	for (;;) {
 		buf = vmalloc(buf_size);