[osh2oil] Fix crash when translating empty here doc.

It seems like has to have another redirect after it.  Example:

git/t/t4015-diff-whitespace.sh

The translation no longer crashes, but it's still wrong.

core/lexer.py

@@ -197,6 +197,9 @@ def _Read(self, lex_mode):
       line_id, line, line_pos = self.line_reader.GetLine()
 
       if line is None:  # no more lines
+        # NOTE: Eof_Real has no contents, but it has a span_id because we want
+        # to retrieve the path and line number in ui.PrettyPrintError().
+        # The line_id might be -1.
         span_id = self.line_lexer.GetSpanIdForEof()
         t = ast.token(Id.Eof_Real, '', span_id)
         return t

tools/osh2oil.py

@@ -38,20 +38,19 @@ def __init__(self, arena, f):
 
   def PrintUntil(self, until_span_id):
     # Sometimes we add +1
-    assert until_span_id < const.NO_INTEGER, 'Missing span ID, got %d' % until_span_id
-    #log('PrintUntil %d', until_span_id)
+    assert until_span_id < const.NO_INTEGER, \
+        'Missing span ID, got %d' % until_span_id
     for span_id in range(self.next_span_id, until_span_id):
-      #log('Looking up span id %d', span_id)
       span = self.arena.GetLineSpan(span_id)
-      #log('SPAN %s', span)
 
-      assert span.line_id != -1, 'Invalid span %d: %s' % (span_id, span)
+      # A span for Eof may have a line_id of -1 when the file is completely
+      # empty.
+      if span.line_id == -1:
+        continue
+
       line = self.arena.GetLine(span.line_id)
       piece = line[span.col : span.col + span.length]
       self.f.write(piece)
-      # Spacing
-      #self.f.write('%r' % piece)
-    #self.f.write('__')
 
     self.next_span_id = until_span_id
 
@@ -1107,7 +1106,6 @@ def DoWordPart(self, node, local_symbols, quoted=False):
     span_id = word.LeftMostSpanForPart(node)
     if span_id is not None and span_id != const.NO_INTEGER:
       span = self.arena.GetLineSpan(span_id)
-      #print(span)
 
       self.cursor.PrintUntil(span_id)