ZJIT: Avoid runtime exceptions from RubyVM::ZJIT.stats_string (ruby#16139)

Before this it would raise if zjit wasn't enabled
and raise a different exception if zjit was but extended stats were
not (_some_ stats are available).

Co-authored-by: Randy Stauner <randy@r4s6.net>

Author: k0kubun

test/ruby/test_zjit.rb

@@ -27,6 +27,18 @@ def test_stats_enabled
     RUBY
   end
 
+  def test_stats_string_no_zjit
+    assert_runs 'nil', <<~RUBY, zjit: false
+      RubyVM::ZJIT.stats_string
+    RUBY
+    assert_runs 'true', <<~RUBY, stats: false
+      RubyVM::ZJIT.stats_string.is_a?(String)
+    RUBY
+    assert_runs 'true', <<~RUBY, stats: true
+      RubyVM::ZJIT.stats_string.is_a?(String)
+    RUBY
+  end
+
   def test_stats_quiet
     # Test that --zjit-stats-quiet collects stats but doesn't print them
     script = <<~RUBY

zjit.rb

@@ -165,13 +165,13 @@ def reset_stats!
 
   # Get the summary of ZJIT statistics as a String
   def stats_string
+    return unless stats = self.stats
     buf = +"***ZJIT: Printing ZJIT statistics on exit***\n"
-    stats = self.stats
 
-    if stats[:guard_type_count].nonzero?
+    if stats[:guard_type_count]&.nonzero?
       stats[:guard_type_exit_ratio] = stats[:exit_guard_type_failure].to_f / stats[:guard_type_count] * 100
     end
-    if stats[:guard_shape_count].nonzero?
+    if stats[:guard_shape_count]&.nonzero?
       stats[:guard_shape_exit_ratio] = stats[:exit_guard_shape_failure].to_f / stats[:guard_shape_count] * 100
     end