Fix crash on 'dodgy' .man file in v3

This is a fix for bug 205 which is registered against v2.1 but
is also present in v3. This fix will not be relevant for v2.1
since the cpi file has been replaced with the cpx file.

src/RideFileCache.cpp

@@ -558,6 +558,10 @@ MeanMaxComputer::run()
         double secs = round(p->secs * 1000.0) / 1000;
         if (secs > 0) data.points.append(cpintpoint(secs, (int) round(p->value(baseSeries))));
     }
+
+    // don't bother with insufficient data
+    if (!data.points.count()) return;
+
     int total_secs = (int) ceil(data.points.back().secs);
 
     // don't allow data more than two days