Fix space flag when Inf/NaN and width==3
* sprintf.c (rb_str_format): while `"% 2f"` and `"% 4f"` result in
`" Inf"` and `" Inf"` respectively, `"% 3f"` results in
`"Inf"` (no space).
Refactor "%f" % Inf/NaN
* sprintf.c (rb_str_format): as for non-finite float, calculate
the exact needed size with the space flag.
merge revision(s) 58453,58454: [Backport #13499]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@59218 b2dd03c8-39d4-4d8f-98ff-823fe69b080e