BUG/MEDIUM: mux-fcgi: Avoid value length overflow when it doesn't fit at once

BUG/MEDIUM: mux-fcgi: Avoid value length overflow when it doesn't fit at once

When the request data are copied in a mbuf, if the free space is too small
to copy all data at once, the data length is shortened. When this is
performed, we reserve the size of the STDIN recod header and eventually the
same for the empty STDIN record if it is the last HTX block of the request.

However, there is no test to be sure the free space is large enough. Thus,
on this special case, when the mbuf is almost full, it is possible to
overflow the value length. Because of this bug, it is possible to experience
crashes from time to time.

This patch should fix the issue #1923. It must be backported as far as 2.4.