merge revision(s) 42905: [Backport #8884]

	* vm_backtrace.c (vm_backtrace_to_ary): Ignore the second argument if
	  it is nil. [Bug #8884] [ruby-core:57094]

	* test/ruby/test_backtrace.rb (test_caller_with_nil_length):
	  Test for above.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@42925 b2dd03c8-39d4-4d8f-98ff-823fe69b080e