fix "unknown SDL joystick axes mapped to 'pad.X Axis-'" bug.

When SDLJoystick::ProcessInput() receives an event on some axis
which is unknown to PPSSPP (accelerometer, for example), the
event should be discarded.  But SDLJoystick::ProcessInput() uses
operator [] to lookup axis map, and it creates a new entry which
maps to 0 (X Axis-), without error.

See http://forums.ppsspp.org/showthread.php?tid=9463 for actual
problem.

This patch modifies SDLJoystick::ProcessInput() to use
find() instead of [] and discard the event if no mapping is
defined for the axis.