[tree] Reset fNamesToBranches map in Streamer()

When reading, the Streamer() function may reuse the Tree. If the
fNamesToBranch map is not invalidated GetBranch() may return stale
pointers to freed memory.

Fixes #20706

(cherry picked from commit 7603459)

Author: silverweed

tree/tree/src/TTree.cxx

@@ -9670,6 +9670,7 @@ void TTree::Streamer(TBuffer& b)
       fDirectory = nullptr;
       fCacheDoAutoInit = true;
       fCacheUserSet = false;
+      fNamesToBranches.clear();
       Version_t R__v = b.ReadVersion(&R__s, &R__c);
       if (R__v > 4) {
          b.ReadClassBuffer(TTree::Class(), this, R__v, R__s, R__c);