sys-devel/flex: fix crash on Apple M1 due to undefined behavior.

Currently, when the NULL-terminated variadic function
filter_create_ext() is invoked, the value "0" is passed as
the last argument to act as a terminator. However, this is
an integer value, which is incompatible with the pointer
data type expected by filter_create_ext().

This is undefined behavior in C, correct operation is not
guaranteed. In fact, it causes flex to crash on Apple M1
when GCC is used - the loop is not terminated when it should,
instead, it keeps running, corrupting the argument list for
invoking m4. As a result, it creates the following error:

> flex: fatal internal error, exec of gm4 failed

This commit fixes the problem by explicitly casting the value
0 to the correct pointer type (char *).

Since the existence of the bug doesn't always prevent a Gentoo
Prefix bootstrapping, it can lurk inside the system and remain
undetected, furthermore, it's technically a C programming bug,
other platforms could've been affected as well in theory. Thus,
we also bump the package version.

Closes: https://bugs.gentoo.org/871324
Signed-off-by: Yifeng Li <tomli@tomli.me>
Signed-off-by: Sam James <sam@gentoo.org>

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