Fix segfault when DMAing past the end of cart ROM.

When a game triggers a DMA from the cart ROM area to RDRAM, part or all of the
DMA'd area is not covered by the currently-loaded ROM, cen64 treats it as a
series of reads from open-bus. A bug in the code that handles this case was
causing segfaults.
Hat-tip to cen64 user Grim who provided a test rom that triggered this bug. His
test ROM was less than (1 MB + 4 KB) in size, which was causing the initial 1MB
DMA to trigger this bug.

pi/controller.c

@@ -138,7 +138,7 @@ static int pi_dma_write(struct pi_controller *pi) {
       for (i = (pi->regs[PI_CART_ADDR_REG] + pi->rom_size + 3) & ~0x3;
           i < pi->regs[PI_CART_ADDR_REG] + length; i += 4) {
         uint32_t word = (i >> 16) | (i & 0xFFFF0000);
-        memcpy(pi->bus->ri->ram + dest + i, &word, sizeof(word));
+        memcpy(pi->bus->ri->ram + dest, &word, sizeof(word));
       }
 
       length = pi->rom_size - source;