Eliminate less-than-zero checks for unsigned variables

* ext/bigdecimal/bigdecimal.c, ext/digest/md5/md5.c,
  ext/json/fbuffer/fbuffer.h, ext/json/generator/generator.c:
  Eliminate less-than-zero checks for unsigned variables.
  According to section 4.1.5 of C89 standard, size_t is an unsigned
  type.  These checks were found with 'cppcheck' static analysis tool.
  [ruby-core:57117] [Feature #8890]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42920 b2dd03c8-39d4-4d8f-98ff-823fe69b080e