load.c: use local variable

* load.c (rb_load_internal): use local variable th instead of
  GET_THREAD() for each time.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43498 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
load.c: suppress warning

* load.c (rb_load_internal): split to suppress false "clobbered"
  warning by gcc 4.4.5.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43499 b2dd03c8-39d4-4d8f-98ff-823fe69b080e