BUG/MEDIUM: h3: do not crash if no buf space for trailers

BUG/MEDIUM: h3: do not crash if no buf space for trailers

Replace ABORT_NOW() by proper error management in
h3_resp_trailers_send() for QPACK encoding operation.

If a QPACK encoding operation fails, it means there is not enough space
in qcs buffer. In this case, flag qcs instance with QC_SF_BLK_MROOM and
return an error. MUX is responsible to remove this flag once buffer
space is available.

This should fix the crash reported by gabrieltz on github issue #2006.

This must be backported up to 2.7.