[flang] Fix a crash with IMPORT in BLOCK (issue 604)

Only call scope.name() if the scope has a symbol (Block, Global,
Forall and ImpliedDo kind of scopes do not have a symbol).

Original-commit: flang-compiler/f18@81c6b67dd2cbb926aba967d9ce2b8973355ca737
Reviewed-on: https://github.com/flang-compiler/f18/pull/634
Tree-same-pre-rewrite: false