Minor micro-optimization, I could not find much improvement while
keeping functionality:

http://80x24.org/spew/m/thread-microopt-v1%40r49282.txt

target 0: 2.1.5 (ruby 2.1.5p273 (2014-11-13 revision 48405) [x86_64-linux]) at "/home/ew/ruby-2.1/bin/ruby"
target 1: trunk (ruby 2.3.0dev (2015-01-16 trunk 49282) [x86_64-linux]) at "/home/ew/rrrr/b/i/bin/ruby"
target 2: built (ruby 2.3.0dev (2015-01-16 trunk 49282) [x86_64-linux]) at "/home/ew/ruby/b/i/bin/ruby"

2.1.5	1.0573025540215895
2.1.5	1.0493981029139832
2.1.5	1.0576379200210795
trunk	1.2876477020327002
trunk	1.2424484699731693
trunk	1.2432217099703848
built	1.1531978889834136
built	1.137529328931123
built	1.1509092160267755

Elapsed time: 10.381246521 (sec)
-----------------------------------------------------------
benchmark results:
minimum results in each 3 measurements.
Execution time (sec)
name	2.1.5	trunk	built
vm_thread_create_join	1.049	1.242	1.138

Speedup ratio: compare with the result of `2.1.5' (greater is better)
name	trunk	built
vm_thread_create_join	0.845	0.923