(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.