signedness bug & integer overflow in _XcursorFileHeaderCreate() [CVE-2013-2003]

When parsing cursor files, a user defined (e.g. through environment
variables) cursor file is opened and parsed.

The header is read in _XcursorReadFileHeader(), which reads an unsigned
int for the number of toc structures in the header, but it was being
passed to _XcursorFileHeaderCreate() as a signed int to allocate those
structures.  If the number was negative, it would pass the bounds check
and could overflow the calculation for how much memory to allocate to
store the data being read, leading to overflowing the buffer with the
data read from the user controlled file.

Reported-by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Signed-off-by: Julien Cristau <jcristau@debian.org>