Fix GCC 14 build failure and simplify XML error handler

[antecrescent]

This was reported downstream at https://bugs.gentoo.org/928331.

The compilation fails with GCC 14 due to incompatible-pointer-types being an error by default.

$ ./autogen.sh
$ ./configure
$ make
Making all in src
make[1]: Entering directory '/root/FORT-validator/src'
make  all-am
make[2]: Entering directory '/root/FORT-validator/src'
gcc -DHAVE_CONFIG_H -I.    -Wall -Wpedantic -std=c99 -D_DEFAULT_SOURCE=1 -D_XOPEN_SOURCE=700 -D_BSD_SOURCE=1 -O2 -g  -I/usr/include/libxml2 -DBACKTRACE_ENABLED -g -O2 -MT xml/fort-relax_ng.o -MD -MP -MF xml/.deps/fort-relax_ng.Tpo -c -o xml/fort-relax_ng.o `test -f 'xml/relax_ng.c' || echo './'`xml/relax_ng.c
xml/relax_ng.c: In function 'relax_ng_parse':
xml/relax_ng.c:117:56: error: passing argument 2 of 'xmlTextReaderSetStructuredErrorHandler' from incompatible pointer type [-Wincompatible-pointer-types]
  117 |         xmlTextReaderSetStructuredErrorHandler(reader, relax_ng_log_str_err,
      |                                                        ^~~~~~~~~~~~~~~~~~~~
      |                                                        |
      |                                                        void (*)(void *, xmlError *) {aka void (*)(void *, struct _xmlError *)}
In file included from ./xml/relax_ng.h:4,
                 from xml/relax_ng.c:1:
/usr/include/libxml2/libxml/xmlreader.h:420:75: note: expected 'xmlStructuredErrorFunc' {aka 'void (*)(void *, const struct _xmlError *)'} but argument is of type 'void (*)(void *, xmlError *)' {aka 'void (*)(void *, struct _xmlError *)'}
  420 |                                                    xmlStructuredErrorFunc f,
      |                                                    ~~~~~~~~~~~~~~~~~~~~~~~^
make[2]: *** [Makefile:2712: xml/fort-relax_ng.o] Error 1
make[2]: Leaving directory '/root/FORT-validator/src'
make[1]: *** [Makefile:791: all] Error 2
make[1]: Leaving directory '/root/FORT-validator/src'
make: *** [Makefile:372: all-recursive] Error 1

Since we don't manipulate any struct members, I believe we can safely take a const xmlError * argument instead, as per libxml2's documentation .

I also believe we can drop the string-termination handling, because C strings end with \0 and therefore ptr[strlen(ptr)-1] is always \0.

[ydahhrk]

Thanks!

[antecrescent]

Thank you for merging my changes! Sorry for breaking support with older libxml2 versions. I did not stop to check for API changes before opening the PR... Thank you for fixing that.