test_sprintf_comb.rb: split tests

	* test/ruby/test_sprintf_comb.rb (test_format_integer),
  (test_format_float): split huge tests by the formats.
	* vsnprintf.c (MAXEXP, MAXFRACT): calculate depending on constants in
	  float.h.

	* vsnprintf.c (BSD_vfprintf): limit length for cvt() to get rid of
	  buffer overflow.  [ruby-core:57023] [Bug #8864]

	* vsnprintf.c (exponent): make expbuf size more precise.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_3@43488 b2dd03c8-39d4-4d8f-98ff-823fe69b080e