[xml] GCC 10 analyzer findings

[Jeffrey Walton <noloader gmail com>]

Hi Everyone,

I'm building libxml2-2.9.10 on Fedora 32 with GCC 10. GCC 10 includes
the analyzer. The analyzer can be enabled by adding -fanalyze to
CFLAGS and LDFLAGS.

The analyzer is producing some use-after-free and double-free findings
on libxml2-2.9.10.

Could someone please make a testing pass on libunistring and take a
look at the findings.

Thanks in advance.