Receiving header block fragments referring to invalid indexes causes a crash

[psalin]

Currently, this can be reproduced by running the unary concurrency test that causes #45. This results in a couple of headers being undefined because of invalid indexes, which in turn causes the trace below:

=ERROR REPORT==== 17-Jan-2021::10:50:34.719500 ===
** State machine <0.731.0> terminating
** Last event = {cast,{recv_h,[{<<":status">>,<<"200">>},
                               undefined,undefined,
                               {<<"user-agent">>,<<"grpc-erlang/0.1.0">>}]}}
** When server state  = {half_closed_local,
                         {stream_state,117,<0.666.0>,
                          {gen_tcp,#Port<0.134>},
                          idle,
                          {[],[]},
                          [{<<":method">>,<<"POST">>},
                           {<<":path">>,
                            <<"/routeguide.RouteGuide/GetFeature">>},
                           {<<":scheme">>,<<"http">>},
                           {<<":authority">>,<<"localhost:8080">>},
                           {<<"grpc-encoding">>,<<"identity">>},
                           {<<"grpc-message-type">>,<<"routeguide.Point">>},
                           {<<"content-type">>,<<"application/grpc+proto">>},
                           {<<"user-agent">>,<<"grpc-erlang/0.9.2">>},
                           {<<"te">>,<<"trailers">>}],
                          undefined,0,false,false,[],[],undefined,false,false,
                          undefined,undefined,
                          #{buffer => <<>>,client_pid => <0.639.0>,
                            ctx =>
                             {ctx,
                              #{grpc_client_method =>
                                 <<"/routeguide.RouteGuide/GetFeature">>},
                              undefined},
                            marshal_fun =>
                             #Fun<routeguide_route_guide_client.0.7492253>,
                            path => <<"/routeguide.RouteGuide/GetFeature">>,
                            service => 'routeguide.RouteGuide',stats => #{},
                            stats_handler => undefined,stream_id => 117,
                            unmarshal_fun =>
                             #Fun<routeguide_route_guide_client.1.7492253>},
                          grpcbox_client_stream,client}}
** Reason for termination = error:function_clause
** Callback mode = state_functions
** Stacktrace =
**  [{h2_stream,'-no_upper_names/1-fun-0-',
                [undefined],
                [{file,"grpcbox/_build/default/lib/chatterbox/src/h2_stream.erl"},
                 {line,828}]},
     {lists,all,2,[{file,"lists.erl"},{line,1213}]},
     {h2_stream,is_valid_headers,2,
                [{file,"grpcbox/_build/default/lib/chatterbox/src/h2_stream.erl"},
                 {line,818}]},
     {h2_stream,half_closed_local,3,
                [{file,"grpcbox/_build/default/lib/chatterbox/src/h2_stream.erl"},
                 {line,589}]},
     {gen_statem,loop_state_callback,11,[{file,"gen_statem.erl"},{line,1159}]},
     {proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,249}]}]