[BABEL] Fix Invalid Read in IsTopTransactionName (#739)

SetTopTransactionName() stores the transaction name in TopTransactionContext
memory. When the transaction ends, this memory is freed but the name pointer
is not set to NULL, leaving a dangling pointer. A subsequent call to
IsTopTransactionName() then reads freed memory via strcmp().

Introduce ResetTopTransactionName() which NULLs out name. This is called from
pltsql_xact_cb to ensure the pointer is reset on every transaction end.

Extension PR: babelfish-for-postgresql/babelfish_extensions#4717

Task: BABEL-5245

Authored-by: Rucha Kulkarni ruchask@amazon.com

Author: RuchaSK1

src/backend/access/transam/xact.c

@@ -6543,6 +6543,12 @@ SetTopTransactionName(const char *name)
 	s->name = MemoryContextStrdup(TopTransactionContext, name);
 }
 
+void
+ResetTopTransactionName(void)
+{
+	CurrentTransactionState->name = NULL;
+}
+
 /*
  * IsTrasactionBlockActive
  * If a transaction block is already in progress

src/include/access/xact.h

@@ -535,6 +535,7 @@ extern bool IsInParallelMode(void);
 
 extern PGDLLEXPORT bool IsTopTransactionName(const char *name);
 extern PGDLLEXPORT void SetTopTransactionName(const char *name);
+extern PGDLLEXPORT void ResetTopTransactionName(void);
 extern PGDLLEXPORT bool IsTransactionBlockActive(void);
 extern PGDLLEXPORT void RollbackAndReleaseSavepoint(const char *name);