Skip variant for unknown types

Reading a single variant didn't skip over the message data
when encountering a type that doesn't exist in the template
parameter list.  Subseqeuent reads then point at the wrong data.

Add a call to sdbus_message_skip in this case.

Change-Id: Ia89baced1530698070f7ca4e47ba16f36c2fcafa
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/sdbusplus/message/read.hpp b/sdbusplus/message/read.hpp
index 275e3a4..621721f 100644
--- a/sdbusplus/message/read.hpp
+++ b/sdbusplus/message/read.hpp
@@ -239,6 +239,7 @@
     template<typename S>
     static void read(sd_bus_message* m, S&& s)
     {
+        sd_bus_message_skip(m, "v");
         s = std::remove_reference_t<S>{};
     }