Add sanity check for dshash entries when reading pgstats file

Add sanity check for dshash entries when reading pgstats file

Not having this check would produce a core dump at startup when running
pgstat_read_statsfile(), in the case where the information of a stats
kind for an entry in the dshash could not be found.  The same check
already happens for fixed-numbered stats and entries that are stored
with their names.  This issue can be seen with custom stats kinds.

Note that this problem can be reproduced what what is in the core code:
- Tweak the test module injection_points to not load the fixed-numbered
stats part, leaving only the variable-numbered stats.
- Create an instance with injection_points defined in
shared_preload_libraries.
- Create a pgstats entry by attaching and running a point.
- Restart the server without shared_preload_libraries.  The startup
process detects that something is wrong and reports a WARNING.

Author: Bertrand Drouvot <[email protected]>
Discussion: https://postgr.es/m/[email protected]