Add an assert() to help verify the return code from the Win32 thread wait function.

FossilOrigin-Name: ed3dc7a89f3416622fcd741ae5fba437929d06d6

src/threads.c

@@ -132,6 +132,7 @@ int sqlite3ThreadJoin(SQLiteThread *p, void **ppOut){
   assert( ppOut!=0 );
   if( p==0 ) return SQLITE_NOMEM;
   rc = sqlite3Win32Wait((HANDLE)p->tid);
+  assert( rc!=WAIT_IO_COMPLETION );
   if( rc==WAIT_OBJECT_0 ) *ppOut = p->pResult;
   sqlite3_free(p);
   return (rc==WAIT_OBJECT_0) ? SQLITE_OK : SQLITE_ERROR;