P2P NFC: Make code easier for static analyzers
len + pos > end comparison here did verify that the length field had a
valid value, but that did not seem to enough to avoid TAINTED_SCALAR
warning. Re-order that validation step to be equivalent "len > end -
pos" to remove these false positives (CID 68116).
Signed-off-by: Jouni Malinen <j@w1.fi>