Fix dividing with infinity bug

When calculating the remaining time for a progress task,
we divide the value delta with the current speed.
If the speed is zero, then the resulting double will
be 'infinity' which will lead to TimeSpan.FromSeconds
throwing.

This commit fixes that bug by setting the speed to 1
if it's 0 when calculating the remaining time.

Closes #169

src/Spectre.Console/Progress/ProgressTask.cs

@@ -265,6 +265,14 @@ namespace Spectre.Console
                     return null;
                 }
 
+                // If the speed is zero, the estimate below
+                // will return infinity (since it's a double),
+                // so let's set the speed to 1 in that case.
+                if (speed == 0)
+                {
+                    speed = 1;
+                }
+
                 var estimate = (MaxValue - Value) / speed.Value;
                 return TimeSpan.FromSeconds(estimate);
             }