In attribute content, don't emit entity references if there are
problems with the entity value. Otherwise some illegal entity values
like

    <!ENTITY a '&#38;#x123456789;'>

would later cause problems like integer overflow.

Make xmlStringLenDecodeEntities return NULL on more error conditions
including invalid char refs and errors from recursive calls. Remove
some fragile error checks based on lastError that shouldn't be
needed now. Clear the entity content in xmlParseAttValueComplex if
an error was found.

Found by OSS-Fuzz. Should fix bug 783052.

Also see https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=3343