6LoWPAN: Fix a whole in the logic of the previous commit. It turns out that...

6LoWPAN:  Fix a whole in the logic of the previous commit.  It turns out that g_uncomp_hdrlen has other usages so it cannot be modified as I was doing.  Instead, I needed to add a separate localt variable, protosize, to keep track of the two usages of g_uncomp_hdrlen.