Previous fix for #2285 (and the duplicate #2624) was missdiagnosed. The
problem stems from a wrong assumption that the number of bytes already
pipelined will be less than maxbytes, with maxbytes beeing the maximum
number of bytes the HTC_RxStuff may need to get a full work unit. That
assumption may fail during the H/1 to H/2 upgrade path where maxbytes
change with the context, or during runtime changing of parameters.

This patch makes HTC_RxStuff not assert if the pipelined data turned out
to exceed maxbytes, but return overflow if we run out of workspace.

(#2624 has received a workaround in the H/2 code that perhaps should be
reverted).