Fixing issue in legend.js I found while investigating

issue #171.  Turns out it's possible for a divide by zero
error to happen.

examples/stackedAreaChart.html

@@ -194,6 +194,7 @@ keyColor = function(d, i) {return colors(d.key)};
 var chart;
 nv.addGraph(function() {
   chart = nv.models.stackedAreaChart()
+               // .width(600).height(500)
                 .useInteractiveGuideline(true)
                 .x(function(d) { return d[0] })
                 .y(function(d) { return d[1] })

src/models/legend.js

@@ -94,6 +94,7 @@ nv.models.legend = function() {
           columnWidths[seriesPerRow] = seriesWidths[seriesPerRow];
           legendWidth += seriesWidths[seriesPerRow++];
         }
+        if (seriesPerRow === 0) seriesPerRow = 1; //minimum of one series per row
 
 
         while ( legendWidth > availableWidth && seriesPerRow > 1 ) {