check zeros

raise when assert_linear_performance measures only zeros to check the following error:

```
  1) Error:
TestRegexp#test_linear_performance:
Test::Unit::ProxyError: nil can't be coerced into Integer
    /home/opc/ruby/src/master/test/ruby/test_regexp.rb:2301:in 'TestRegexp#test_linear_performance'
```

1 files changed, 3 insertions, 2 deletions

diff --git a/tool/lib/core_assertions.rb b/tool/lib/core_assertions.rb
index e29a0e3c25..5ca318a598 100644
--- a/tool/lib/core_assertions.rb
+++ b/tool/lib/core_assertions.rb
@@ -911,10 +911,11 @@ eom
 
         first = seq.first
         *arg = pre.call(first)
-        times = (0..(rehearsal || (2 * first))).map do
+        raw_times = (0..(rehearsal || (2 * first))).map do
           measure[arg, "rehearsal"].nonzero?
         end
-        times.compact!
+        times = raw_times.compact
+        raise "all measurements are zero: #{raw_times.inspect}" if times.empty?
         tmin, tmax = times.minmax
 
         # safe_factor * tmax * rehearsal_time_variance_factor(equals to 1 when variance is small)