cover: Fix a race condition in how the server is started as part of a…

… call

[Why]
Before sending a message to the cover server, the `call` function checks
if the server is running, starting it if it does not.

However, there is no locking/atomicity between the check and the start.
If there is a concurrent call, the server might be started in parallel
by another process and the `start()` call would return
`{already_started, _}`. This causes the function to fail with the
`badmatch` exception.

[How]
The code now accepts that the server is running after seeing the server
as stopped and retrying the call.

lib/tools/src/cover.erl

@@ -1143,7 +1143,10 @@ call(Request) ->
     Ref = erlang:monitor(process,?SERVER),
     receive {'DOWN', Ref, _Type, _Object, noproc} -> 
 	    erlang:demonitor(Ref),
-            {ok,_} = start(),
+            ok = case start() of
+                     {ok,_} -> ok;
+                     {'already_started',_} -> ok
+                 end,
 	    call(Request)
     after 0 ->
 	    ?SERVER ! {self(),Request},