Correct fix for "GFP: Fix Dead Store (Dead assignement/Dead increment) Warning found by Clang"

This is the correct fix commit 0e7b7dc883.
(As committed, deleting these two lines makes the dissector return the wrong
number of bytes processed.)

Change-Id: I2e88bbc6942f6af40199d5b03a71e182c1896871
Reviewed-on: https://code.wireshark.org/review/13074
Reviewed-by: Anders Broman <a.broman58@gmail.com>

epan/dissectors/packet-gfp.c

@@ -379,6 +379,8 @@ dissect_gfp_payload(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, proto_t
         default:
             break;
     }
+    *offset += payload_len;
+    *offset += fcs_len;
 }
 
 static int