Call error callback when invalid header field is received and ignored

We have a code to call error callback when invalid header is received
and it is treated as stream error.  But we didn't if the incoming
header is invalid, but just ignored.  This generosity is required to
handle public Internet connections especially when nghttp2 is used as
forward proxy.

lib/nghttp2_session.c

@@ -3604,11 +3604,24 @@ static int inflate_header_block(nghttp2_session *session, nghttp2_frame *frame,
         }
 
         if (rv == NGHTTP2_ERR_IGN_HTTP_HEADER) {
+          /* Don't overwrite rv here */
+          int rv2;
           /* header is ignored */
           DEBUGF(fprintf(
               stderr, "recv: HTTP ignored: type=%u, id=%d, header %.*s: %.*s\n",
               frame->hd.type, subject_stream->stream_id, (int)nv.name->len,
               nv.name->base, (int)nv.value->len, nv.value->base));
+
+          rv2 = session_call_error_callback(
+              session,
+              "Ignoring received invalid HTTP header field: frame type: "
+              "%u, stream: %d, name: [%.*s], value: [%.*s]",
+              frame->hd.type, frame->hd.stream_id, (int)nv.name->len,
+              nv.name->base, (int)nv.value->len, nv.value->base);
+
+          if (nghttp2_is_fatal(rv2)) {
+            return rv2;
+          }
         }
       }
       if (rv == 0) {