BUG/MINOR: http-ext: always check remaining data when reading rfc7239 nodeport

BUG/MINOR: http-ext: always check remaining data when reading rfc7239 nodeport

http_7239_extract_nodeport() reads the first byte of the passed string
but the caller doesn't check that it's not empty, which can happen if
passed as 'host="127.0.0.1:"'. In that case the function would read and
return garbage that is present in the buffer after the colon. Let's just
check the remaining length before reading.

This can be backported to 2.8 as it was introduced with commit b2bb9257d2
("MINOR: proxy/http_ext: introduce proxy forwarded option").