BUG/MEDIUM: mux-fcgi: Properly handle return value of headers/trailers parsing

BUG/MEDIUM: mux-fcgi: Properly handle return value of headers/trailers parsing

h1_parse_msg_hdrs() and h1_parse_msg_tlrs() may return negative values if
the parsing fails or if more space is needed in the destination buffer. When
h1-htx was changed, The H1 mux was updated accordingly but not the FCGI
mux. Thus if a negative value is returned, it is ignored and it is casted to
a size_t, leading to an integer overflow on the <ofs> value, used to know
the position in the RX buffer.

This patch must be backported as far as 2.2.