Fix a crash on dicts with paren-wrapped long string keys (#3262)

Fix a crash when formatting some dicts with parenthesis-wrapped long
string keys. When LL[0] is an atom string, we need to check the atom
node's siblings instead of LL[0] itself, e.g.:

    dictsetmaker
      atom
        STRING '"This is a really long string that can\'t be expected to fit in one line and is used as a nested dict\'s key"'
      /atom
      COLON ':'
      atom
        LSQB ' ' '['
        listmaker
          STRING '"value"'
          COMMA ','
          STRING ' ' '"value"'
        /listmaker
        RSQB ']'
      /atom
      COMMA ','
    /dictsetmaker