Don't throw errors when folding comments

Previously trying to fold single line comments at the start/end
of files would throw errors since the +1 and -1 used in the
index ranges would cause the loop to advance past the last row
or before the first row.

spec/language-mode-spec.coffee

@@ -70,6 +70,28 @@ describe "LanguageMode", ->
         expect(languageMode.rowRangeForCodeFoldAtBufferRow(2)).toBeNull()
         expect(languageMode.rowRangeForCodeFoldAtBufferRow(4)).toEqual [4, 7]
 
+    describe ".rowRangeForCommentAtBufferRow(bufferRow)", ->
+      it "returns the start/end rows of the foldable comment starting at the given row", ->
+        buffer.setText("//this is a multi line comment\n//another line")
+        expect(languageMode.rowRangeForCommentAtBufferRow(0)).toEqual [0, 1]
+        expect(languageMode.rowRangeForCommentAtBufferRow(1)).toEqual [0, 1]
+
+        buffer.setText("//this is a multi line comment\n//another line\n//and one more")
+        expect(languageMode.rowRangeForCommentAtBufferRow(0)).toEqual [0, 2]
+        expect(languageMode.rowRangeForCommentAtBufferRow(1)).toEqual [0, 2]
+
+        buffer.setText("//this is a multi line comment\n\n//with an empty line")
+        expect(languageMode.rowRangeForCommentAtBufferRow(0)).toBeUndefined()
+        expect(languageMode.rowRangeForCommentAtBufferRow(1)).toBeUndefined()
+        expect(languageMode.rowRangeForCommentAtBufferRow(2)).toBeUndefined()
+
+        buffer.setText("//this is a single line comment\n")
+        expect(languageMode.rowRangeForCommentAtBufferRow(0)).toBeUndefined()
+        expect(languageMode.rowRangeForCommentAtBufferRow(1)).toBeUndefined()
+
+        buffer.setText("//this is a single line comment")
+        expect(languageMode.rowRangeForCommentAtBufferRow(0)).toBeUndefined()
+
     describe "suggestedIndentForBufferRow", ->
       it "returns the suggested indentation based on auto-indent/outdent rules", ->
         expect(languageMode.suggestedIndentForBufferRow(0)).toBe 0

src/language-mode.coffee

@@ -148,15 +148,20 @@ class LanguageMode
     return unless @editor.displayBuffer.tokenizedBuffer.tokenizedLineForRow(bufferRow).isComment()
 
     startRow = bufferRow
-    for currentRow in [bufferRow-1..0]
-      break if @buffer.isRowBlank(currentRow)
-      break unless @editor.displayBuffer.tokenizedBuffer.tokenizedLineForRow(currentRow).isComment()
-      startRow = currentRow
     endRow = bufferRow
-    for currentRow in [bufferRow+1..@buffer.getLastRow()]
-      break if @buffer.isRowBlank(currentRow)
-      break unless @editor.displayBuffer.tokenizedBuffer.tokenizedLineForRow(currentRow).isComment()
-      endRow = currentRow
+
+    if bufferRow > 0
+      for currentRow in [bufferRow-1..0]
+        break if @buffer.isRowBlank(currentRow)
+        break unless @editor.displayBuffer.tokenizedBuffer.tokenizedLineForRow(currentRow).isComment()
+        startRow = currentRow
+
+    if bufferRow < @buffer.getLastRow()
+      for currentRow in [bufferRow+1..@buffer.getLastRow()]
+        break if @buffer.isRowBlank(currentRow)
+        break unless @editor.displayBuffer.tokenizedBuffer.tokenizedLineForRow(currentRow).isComment()
+        endRow = currentRow
+
     return [startRow, endRow] if startRow isnt endRow
 
   rowRangeForCodeFoldAtBufferRow: (bufferRow) ->