[flang-rt] Fixed uninitialized class member variable (llvm#127681)

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_;
 };