Fix pathological performance when outputting charrefs
If a character can't be represented in the output encoding, it is converted to a character reference. This used to to replace the character in the input stream by calling xmlBufAddHead or xmlBufferAddHead. These functions shifted the entire input array around, leading to quadratic performance when converting a run of non-representable characters. This is most pronounced when dumping to memory. Output the charref directly instead. Found with libFuzzer.
Showing with 59 additions and 70 deletions