Rework HTTP/2 closed connection logic and consolidate http2 mem_recv() calls in one place.

[laramiel]

Previously there were several locations that called
nghttp2_session_mem_recv and handled responses slightly differently.
Those have been converted to call the existing
h2_process_pending_input() function.

Moved the end-of-session check to h2_process_pending_input() since
the only place the end-of-session state can change is after nghttp2
processes additional input frames.

This will likely fix the fuzzing error. While I don't have a root
cause the out-of-bounds read seems like a use after free, so moving
the nghttp2_session_check_request_allowed() call to a location with
a guaranteed nghttp2 session seems reasonable.

Also updated a few nghttp2 callsites to include error messages and
added a few additional error checks.

This attempts to address #5646

[bagder]

http2 warnings:

http2.c: In function ‘http2_recv’:
http2.c:1659:11: error: variable ‘inbuf’ set but not used [-Werror=unused-but-set-variable]
     char *inbuf;
           ^~~~~
http2.c:1538:11: error: unused variable ‘rv’ [-Werror=unused-variable]
   ssize_t rv;
           ^~
http2.c: In function ‘Curl_http2_switched’:
http2.c:2178:11: error: unused variable ‘nproc’ [-Werror=unused-variable]
   ssize_t nproc;
           ^~~~~
cc1: all warnings being treated as errors

[bagder]

checksrc warnings:

./http2.c:1334:13: warning: space before close parenthesis (SPACEBEFORECLOSE)
   if(rv < 0 ) {
             ^
./http2.c:1695:85: warning: Longer than 79 columns (LONGLINE)
       H2BUGF(infof(data, "Use data left in connection buffer, nread=%zd\n", nread));
./http2.c:2052:58: warning: no space before asterisk (ASTERISKNOSPACE)
          nghttp2_session_check_request_allowed(h2), (void*)conn->data));
                                                          ^
./http2.c:2078:83: warning: Longer than 79 columns (LONGLINE)
     H2BUGF(infof(conn->data, "http2_send() nghttp2_submit_request error (%s)%d\n",
./http2.c:2092:81: warning: Longer than 79 columns (LONGLINE)
     H2BUGF(infof(conn->data, "http2_send() nghttp2_session_send error (%s)%d\n",
checksrc: 0 errors and 5 warnings
checksrc: 0 errors and 6 warnings suppressed
Makefile:4260: recipe for target 'checksrc' failed

[laramiel]

Ping on this. I can move the close connection back, but I think that it makes sense to consolidate the receive logic into a single method.

[bagder]

I'm slow this week, I'll review/merge this soon.

[bagder]

Thanks @laramiel, sorry for taking forever to merge this!