Close #5675

Because ExecuteCommand allocates temporary string space on the stack,
proportional to the length of the command to execute, a stack overflow can
occur when the the command is too long.

This can be triggered remotely e.g. from the VLC Web interface, by
running this JavaScript:

sendVLMCmd(Array.prototype.join.call({length:300000},'a'));

which sends a string of length 300000 to ExecuteCommand, crashing VLC.
OKed-by: Rafaël Carré <funman@videolan.org>
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>