(Hopefully) fix an assert failure in httpinput.cpp.

(Hopefully) fix an assert failure in httpinput.cpp.

If we get the Metacube sync marker but not the right of the header,
we can set has_metacube_header, and then return. If the connection
drops right there, the flag won't be correctly reset, and if the
next connection starts with something that's not a Metacube block,
we will try to read junk data as Metacube.

Thankfully, the assert caught this case.