ShellPkg: Fix the bug that handling Ctrl-C improperly

Current implementation resets the CTRL-C event early when printing
the shell prompt, when user types "<CTRL-C>ls<ENTER>", "ls" command
is terminated immediately when starts.
It's not an expected behavior from users' perspective.
Correct way is to reset the CTRL-C event just before running the
command, which is a bit later than current point.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>