vsprintf: Handle NULL with %pU

At present a NULL pointer passed to printf for a %pU argument will cause
U-Boot to access memory at 0. Fix this by adding a check, and print
"(null)" instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alexander Graf <agraf@suse.de>
[agraf: s/(null)/<NULL>/]
Signed-off-by: Alexander Graf <agraf@suse.de>

lib/vsprintf.c

@@ -407,7 +407,10 @@ static char *uuid_string(char *buf, char *end, u8 *addr, int field_width,
 		break;
 	}
 
-	uuid_bin_to_str(addr, uuid, str_format);
+	if (addr)
+		uuid_bin_to_str(addr, uuid, str_format);
+	else
+		strcpy(uuid, "<NULL>");
 
 	return string(buf, end, uuid, field_width, precision, flags);
 }