If the reported number of motion events is too large, the calculations
to allocate memory for them may overflow, leaving us writing beyond the
bounds of the allocation.

v2: Ensure nEvents is set to 0 when returning NULL events pointer
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>
Backported-to-NX-by: Ulrich Sibiller <uli42@gmx.de>