[flang-rt] Fixed uninitialized class member variable

valgrind complained that `OpenStatementState::pathLength_` was used before it
was initialized, when a file was opened with `status='scratch'`. The code
seems to expect that `pathLengh_` should be initialized to 0, so added default
initialization to the declaration.

Author: eugeneepshteyn

flang-rt/include/flang-rt/runtime/io-stmt.h

@@ -627,7 +627,7 @@ class OpenStatementState : public ExternalIoStatementBase {
   Fortran::common::optional<Action> action_;
   Convert convert_{Convert::Unknown};
   OwningPtr<char> path_;
-  std::size_t pathLength_;
+  std::size_t pathLength_{};
   Fortran::common::optional<bool> isUnformatted_;
   Fortran::common::optional<Access> access_;
 };