don't initialize variables if the first thing we do is assign a
different value to them

Change-Id: I719db9180b2ba5f21653086f2697ca9bac68d6b1
Reviewed-on: https://code.wireshark.org/review/5652
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>