use NUM2SIZET instead of NUM2INT

* ext/dl/dl.c (rb_dl_realloc): use NUM2SIZET instead of NUM2INT.

* ext/fiddle/fiddle.c (rb_fiddle_realloc): ditto.

* ext/dl/dl.c (rb_dl_malloc): use NUM2SIZET instead of NUM2INT.
  Coverity Scan found this bug.

* ext/fiddle/fiddle.c (rb_fiddle_malloc): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45174 b2dd03c8-39d4-4d8f-98ff-823fe69b080e