At last, a fix for `large-clipboard-crash'.

A growable buffer was only being grown for actual text, not for newlines or
trailing NULs. A large run of empty lines could lead to newlines overflowing
the buffer (> 100 should be enough to guarantee this on all platforms, after
the initial 5k size of the buffer).

Also fix some valgrind in the same area (was probably harmless), and a memory
leak introduced by the RTF attribute pasting.

git-svn-id: svn://svn.tartarus.org/sgt/putty@6570 cda61777-01e9-0310-a592-d414129be87e