[xml] clang reports a few problems in libxml2

[Jim Meyering <jim meyering net>]

On request,

I ran clang against libxml2.  The result is a nice html summary,
but the total weighs in at 220MB, uncompressed.  Here's the browsable
result:

  http://meyering.net/code/tmp/clang/libxml2-vs-clang-syntax-checker/index.html

you might want that hierarchy local.
compressed it's just 1.8MiB:

  http://meyering.net/code/tmp/clang/libxml2-vs-clang-syntax-checker.tar.xz

You'll need xz (from xz-utils http://tukaani.org/xz/) to uncompress that.

Note that some of these are definitely false positives.

But here's a legitimate bug in xmlslave.c:

  http://meyering.net/code/tmp/clang/libxml2-vs-clang-syntax-checker/report-5Qxdd7.html#EndPath

    if (doc != NULL)
        oldenc = doc->encoding;                   // one-line "then" clause
        if (ctxt->encoding != NULL) {             // not part of "if block"
            doc->encoding = BAD_CAST ctxt->encoding;
        } else if (doc->encoding != NULL) {
            encoding = doc->encoding;
        }
    }