Fix issue #135: Segfault in Message::peekType()

* Add missing nullptr check in Message::peekType().

* According to its specification, sd_bus_message_peek_type() sets a
  given contents parameter to NULL if the next element in a message is
  not a container. Since assigning a nullptr to a std::string has
  undefined behaviour (typically resulting in an invalid memory access),
  Message::peekType() must not assign contentsSig unconditionally.

src/Message.cpp

@@ -617,7 +617,7 @@ void Message::peekType(std::string& type, std::string& contents) const
     auto r = sd_bus_message_peek_type((sd_bus_message*)msg_, &typeSig, &contentsSig);
     SDBUS_THROW_ERROR_IF(r < 0, "Failed to peek message type", -r);
     type = typeSig;
-    contents = contentsSig;
+    contents = contentsSig ? contentsSig : "";
 }
 
 bool Message::isValid() const