Fixes zero division when there are no lines to plot (only one revision in range)

(SkipBuildbotRuns)
Review URL: https://codereview.chromium.org/16907003

git-svn-id: http://skia.googlecode.com/svn/trunk@9578 2bbb7eff-a529-9590-31e7-b0007b416f81

bench/bench_graph_svg.py

@@ -819,6 +819,10 @@ def output_svg(lines, regressions, requested_width, requested_height):
     font_size = 16
     line_width = 2
 
+    # If there is nothing to see, don't try to draw anything.
+    if w == 0 or h == 0:
+        return
+
     pic_width, pic_height = compute_size(requested_width, requested_height
                                        , w, h)