string.c: avoid signed integer overflow

The behavior on signed integer overflow is undefined. On platform with
sizeof(long)==4, it's fairly easy that 'len + termlen' overflows, where
len is the string length and termlen is the terminator length.

So, prevent the integer overflow by avoiding adding to a string length,
or casting to size_t before adding where the total size is passed to
{RE,}ALLOC*().

* string.c (STR_HEAP_SIZE, RESIZE_CAPA_TERM, str_new0, rb_str_buf_new,
  str_shared_replace, rb_str_init, str_make_independent_expand,
  rb_str_resize): Avoid overflow by casting the length to size_t. size_t
  should be able to represent LONG_MAX+termlen.

* string.c (rb_str_modify_expand): Check that the new length is in the
  range of long before resizing. Also refactor to use RESIZE_CAPA_TERM
  macro.

* string.c (str_buf_cat): Fix so that it does not create a negative
  length String. Also fix the condition for 'string sizes too big', the
  total length can be up to LONG_MAX.

* string.c (rb_str_plus): Check the resulting String length does not
  exceed LONG_MAX.

* string.c (rb_str_dump): Fix integer overflow. The dump result will be
  longer then the original String.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56157 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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