Merge pull request #6087 from hzhou/2207_nth_pset

init: fix error checking in  MPI_Session_get_nth_pset

Approved-by: Ken Raffenetti

Author: hzhou

src/mpi/errhan/errnames.txt

@@ -10,6 +10,7 @@
 **finalized:MPI has already called MPI_Finalize
 **inittwice:Cannot call MPI_INIT or MPI_INIT_THREAD more than once
 **psetinvalidn:Invalid pset number
+**psetinvalidn %d:Invalid pset number - %d
 **psetinvalidname:Invalid pset name
 **session:Invalid MPI_Session handle
 **sessionnull: NULL MPI_Session

src/mpi/init/init_impl.c

@@ -194,8 +194,9 @@ int MPIR_Session_get_nth_pset_impl(MPIR_Session * session_ptr, MPIR_Info * info_
         i++;
     }
 
-    if (i != n) {
-        MPIR_ERR_SETANDSTMT(mpi_errno, MPI_ERR_ARG, goto fn_fail, "**psetinvalidn");
+    if (!MPIR_pset_list[i]) {
+        MPIR_ERR_SETANDSTMT1(mpi_errno, MPI_ERR_ARG, goto fn_fail, "**psetinvalidn",
+                             "**psetinvalidn %d", n);
     }
 
     int len = strlen(MPIR_pset_list[i]);