Do not destroy the strpool if NULL

Do not destroy the strpool if NULL
It is possible that a debug info contains no string (and so strpool
is never allocated).
A protection to avoid accessing strpool was already necessary
in ML_(canonicaliseTables) :
   if (di->strpool)
      VG_(freezeDedupPA) (di->strpool);

So, if a similar debug info is released, we need the same protection
to avoid accessing a NULL strpool.

Detect by Julian on arm64, but not (at least easily) reproduced on amd64.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14033