Only allow positive widths in characters to be assigned

Fixes #1574

spec/display-buffer-spec.coffee

@@ -200,6 +200,12 @@ describe "DisplayBuffer", ->
         expect(tokensText displayBuffer.lineForRow(12).tokens).toBe 'sort(left).concat(pivot).concat(sort(rig'
         expect(changeHandler).toHaveBeenCalledWith(start: 0, end: 15, screenDelta: 3, bufferDelta: 0)
 
+      it "only allows positive widths to be assigned", ->
+        displayBuffer.setEditorWidthInChars(0)
+        expect(displayBuffer.editorWidthInChars).not.toBe 0
+        displayBuffer.setEditorWidthInChars(-1)
+        expect(displayBuffer.editorWidthInChars).not.toBe -1
+
   describe "primitive folding", ->
     beforeEach ->
       displayBuffer.destroy()

src/display-buffer.coffee

@@ -99,10 +99,11 @@ class DisplayBuffer extends Model
   #
   # editorWidthInChars - A {Number} of characters.
   setEditorWidthInChars: (editorWidthInChars) ->
-    previousWidthInChars = @editorWidthInChars
-    @editorWidthInChars = editorWidthInChars
-    if editorWidthInChars isnt previousWidthInChars and @softWrap
-      @updateWrappedScreenLines()
+    if editorWidthInChars > 0
+      previousWidthInChars = @editorWidthInChars
+      @editorWidthInChars = editorWidthInChars
+      if editorWidthInChars isnt previousWidthInChars and @softWrap
+        @updateWrappedScreenLines()
 
   getSoftWrapColumn: ->
     if atom.config.get('editor.softWrapAtPreferredLineLength')