Fix failure on progress bar render (#67)

Fixes two potential runtime errors:
- `@cursor` overshooting `tests_length` which leads to a
	negatively-sized "unfinished" portion of the bar
- running quickdraw in a directory with no tests, where
	`progress = (0.0 / 0)` resolves to `Float::NaN`

Author: z1lk

lib/quickdraw/runner.rb

@@ -209,7 +209,7 @@ def supervise(worker)
 					end
 
 					if console
-						progress = (@cursor * 100.0 / tests_length)
+						progress = @cursor >= tests_length ? 100.0 : (@cursor * 100.0 / tests_length)
 						print "\r\e[K#{'█' * (progress * bar_width / 100.0).floor}#{'░' * (bar_width - (progress * bar_width / 100.0).floor)} #{progress.round}%"
 					end
 				when Message::Stopping