dtdocbook/instant: fix buffer overlow on German umlaut in latin-1

dtdocbook/instant: fix buffer overlow on German umlaut in latin-1

One-off error:

Old buffer length was 6 for one character (3 * 1 + 3)
We need one more byte par character in the buffer for
the hex representation of it.

+0 '"'
+1 '\\'
+2 'x'
+3 'f'
+4 'c'
+5 '"'
+6 0x0 << overflow

tcl combined with RCHECK will abort because memory blocks
are allocated contiguously and we overwrite the magic marker
of the next block.