xfree can handle null values, so we don't need to check it.

A warning for this is shown when `ONIG_DEBUG_COMPILE` is enabled.

[Bug #20161]

The cc->mbuf gets overwritten, so we need to free it to not leak memory.

For example:

    str = "hello world".encode(Encoding::UTF_32LE)

    10.times do
      1_000.times do
        str.grapheme_clusters
      end

      puts `ps -o rss= -p #{$$}`
    end

Before:

    15536
    15760
    15920
    16144
    16304
    16480
    16640
    16784
    17008
    17280

After:

    15584
    15584
    15760
    15824
    15888
    15888
    15888
    15888
    16048
    16112

Apply Nobu's suggestions which improve style, memory handling and error correction.

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

before this change, creating a regex from a regex with a named capture, Regexp.new(/(?<name>)/), causes memory to be shared between the two named capture groups which can cause a segfault if the original is GCed.

Co-authored-by: Adam Hess <HParker@github.com>

According to the C99 specification section 7.20.3.2 paragraph 2:

> If ptr is a null pointer, no action occurs.

So we do not need to check that the pointer is a null pointer.

A code pattern `p + enclen(enc, p, pend)` may lead to a buffer overrun
if incomplete bytes of a UTF-8 character is placed at the end of a
string. Because this pattern is used in several places in onigmo,
this change fixes the issue in the side of `enclen`: the function should
not return a number that is larger than `pend - p`.

Co-Authored-By: Nobuyoshi Nakada <nobu@ruby-lang.org>