Minor pppoe cleanup

cppcheck said:
[decode-pppoe.c:58] -> [decode-pppoe.c:60]: (performance, inconclusive) Variable 'pppoedh' is reassigned a value before the old one has been used if variable is no semaphore variable.

src/decode-pppoe.c

@@ -55,7 +55,6 @@ void DecodePPPOEDiscovery(ThreadVars *tv, DecodeThreadVars *dtv, Packet *p, uint
         ENGINE_SET_EVENT(p, PPPOE_PKT_TOO_SMALL);
         return;
     }
-    p->pppoedh = NULL;
 
     p->pppoedh = (PPPOEDiscoveryHdr *)pkt;
     if (p->pppoedh == NULL)