Match in diameter_app_SUITE to identify an unexpected path

See this failure in nightly test on one host, indicating that
code:which/1 is returning something other than the expected path:

=== Ended at 2018-06-08 05:10:41
=== Location: [{lists,nth,170},
              {diameter_app_SUITE,app,265},
              {diameter_app_SUITE,'-xref/1-fun-3-',214},
              {lists,'-filter/2-lc$^0/1-0-',1286},
              {diameter_app_SUITE,xref,214},
              {test_server,ts_tc,1545},
              {test_server,run_test_case_eval1,1063},
              {test_server,run_test_case_eval,995}]
=== Reason: no function clause matching lists:nth(2,[]) (lists.erl, line 170)
  in function  diameter_app_SUITE:app/1 (diameter_app_SUITE.erl, line 265)
  in call from diameter_app_SUITE:'-xref/1-fun-3-'/2 (diameter_app_SUITE.erl, line 214)
  in call from lists:'-filter/2-lc$^0/1-0-'/2 (lists.erl, line 1286)
  in call from diameter_app_SUITE:xref/1 (diameter_app_SUITE.erl, line 214)
  in call from test_server:ts_tc/3 (test_server.erl, line 1545)
  in call from test_server:run_test_case_eval1/6 (test_server.erl, line 1063)
  in call from test_server:run_test_case_eval/9 (test_server.erl, line 995)

1 files changed, 4 insertions, 2 deletions

diff --git a/lib/diameter/test/diameter_app_SUITE.erl b/lib/diameter/test/diameter_app_SUITE.erl
index 71256020f5..3c1313c296 100644
--- a/lib/diameter/test/diameter_app_SUITE.erl
+++ b/lib/diameter/test/diameter_app_SUITE.erl
@@ -1,7 +1,7 @@
 %%
 %% %CopyrightBegin%
 %%
-%% Copyright Ericsson AB 2010-2015. All Rights Reserved.
+%% Copyright Ericsson AB 2010-2018. All Rights Reserved.
 %%
 %% Licensed under the Apache License, Version 2.0 (the "License");
 %% you may not use this file except in compliance with the License.
@@ -262,7 +262,9 @@ app(Mod) ->
         preloaded ->
             "erts";
         Path ->
-            unversion(lists:nth(3, lists:reverse(filename:split(Path))))
+            %% match to identify an unexpectedly short path
+            {_, _, [_,_,_|_] = Split} = {Mod, Path, filename:split(Path)},
+            unversion(lists:nth(3, lists:reverse(Split)))
     end.
 
 add_application(XRef, App) ->