THRIFT-4842 - Fix memory leak on set property c_glib (#1773)

diff --git a/lib/c_glib/src/thrift/c_glib/protocol/thrift_multiplexed_protocol.c b/lib/c_glib/src/thrift/c_glib/protocol/thrift_multiplexed_protocol.c
index 352e301..5084ead 100644
--- a/lib/c_glib/src/thrift/c_glib/protocol/thrift_multiplexed_protocol.c
+++ b/lib/c_glib/src/thrift/c_glib/protocol/thrift_multiplexed_protocol.c
@@ -76,6 +76,7 @@
   switch (property_id)
   {
   case PROP_THRIFT_MULTIPLEXED_PROTOCOL_SERVICE_NAME:
+    g_free(self->service_name);
     self->service_name = g_value_dup_string (value);
     break;