cg3: add extra check to prevent CG3 register array overflow

The case statements in the CG3 read and write register routines have a maximum
value of CG3_REG_SIZE, so if a value were written to this offset then it
would overflow the register array.

Currently this cannot be exploited since the MemoryRegion restricts accesses
to the range 0 ... CG3_REG_SIZE - 1, but it seems worth clarifying this for
future review and/or static analysis.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
CC: Paolo Bonzini <pbonzini@redhat.com>
