vsprintf.c: correct printing of a NULL device path

When printing '%pD' with a value of NULL we want to output
'<NULL>'. But this requires copying to buf. Leave this
to string16.

A unit test is supplied which relies on EFI support in the sandbox.

The development for EFI support in the sandbox is currently in branch
u-boot-dm/efi-working. The branch lacks commit 6ea8b580f06b ("efi_loader:
correct DeviceNodeToText for media types"). Ater rebasing the aforementioned
branch on U-Boot v2018.01 and adding 256060e4257a2 and this patch the test
is executed successfully.

Fixes: 256060e4257a2 (vsprintf.c: add EFI device path printing)
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>

lib/vsprintf.c

@@ -300,8 +300,9 @@ static char *device_path_string(char *buf, char *end, void *dp, int field_width,
 {
 	u16 *str;
 
+	/* If dp == NULL output the string '<NULL>' */
 	if (!dp)
-		return "<NULL>";
+		return string16(buf, end, dp, field_width, precision, flags);
 
 	str = efi_dp_str((struct efi_device_path *)dp);
 	if (!str)

test/print_ut.c

@@ -44,6 +44,10 @@ static void efi_ut_print(void)
 
 	snprintf(str, sizeof(str), "_%pD_", buf);
 	assert(!strcmp("_/SD(3)_", str));
+
+	/* NULL device path */
+	snprintf(str, sizeof(str), "_%pD_", NULL);
+	assert(!strcmp("_<NULL>_", str));
 #endif
 }