bignum.c: suppress an empty-body warning

	* bignum.c (bigdivrem): move decrement to the loop body, to suppress
  an empty-body warning.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_3@41642 b2dd03c8-39d4-4d8f-98ff-823fe69b080e