Core: Make CPU_POWERDOWN the initial CPU state.

This isn't a correct state for the CPU to begin in when starting the application.

Also CPU_STEPPING as an initial state causes the emulator to crash if you use any of the debugger stepping buttons, since it checks if the CPU is in the CPU_STEPPING state before performing their functions.
DolphinWX: Fix a possible crash when setting debugger memory values.
Merge pull request #655 from lioncash/crashes

Fix two possible crashes related to the debugger.