math.c: fix for Bignum argument

* math.c (math_log, math_log2, math_log10): fix for Bignum argument.
  numbits should be add only when right shifted.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43080 b2dd03c8-39d4-4d8f-98ff-823fe69b080e