Fix null-pointer crash in TimeSeriesManager%get_time_series

BndTsHashTable is only allocated by HashBndTimeSeries(), which
tsmanager_df() calls only when numtsfiles > 0. In a simulation with
no TS6 files, the hash table stays unassociated. If a list-input
value that should be numeric or a time-series name fails to parse as
numeric (e.g. a malformed AUX or boundname column), get_time_series()
dereferences that null pointer directly, crashing with SIGSEGV instead
of falling through to the intended "Expected numeric value or
time-series name, but found '...'" error.

Guard the hash-table lookup with an associated() check, matching the
existing pattern in the sibling GetLink() function in this same file.
With no TS files loaded, no time-series name can match by definition,
so returning "not found" is exactly correct -- this lets the intended
error path fire instead of crashing.

Verified with a debug build (-fcheck=all): before the fix, a
malformed LAK PACKAGEDATA aux value reproducibly segfaults; after,
MF6 reports the expected input-parsing error and terminates cleanly.

Author: wpbonelli

src/Utilities/TimeSeries/TimeSeriesManager.f90

@@ -463,9 +463,11 @@ function get_time_series(this, name) result(res)
     ! Get index from hash table, get time series from TsContainers,
     !     and assign result to time series contained in link.
     res => null()
-    indx = this%BndTsHashTable%get(name)
-    if (indx > 0) then
-      res => this%TsContainers(indx)%timeSeries
+    if (associated(this%BndTsHashTable)) then
+      indx = this%BndTsHashTable%get(name)
+      if (indx > 0) then
+        res => this%TsContainers(indx)%timeSeries
+      end if
     end if
   end function get_time_series