Initialize ir->branch_table->PC with safe value

If the ra(return address) is 0x0, the
LOOKUP_OR_UPDATE_BRANCH_HISTORY_TABLE will bahave abnormally since
calloc initializes ir->branch_table->PC[i] to 0x0. The address 0x0 might
be not yet translated to a valid block, thus ir->branch_table->target[i]
might be NULL, accessing a NULL pointer causes segmentation fault. It
can be solved by initializing ir->branch_table->PC with other value than
0x0. Here, I choose unsigned integer of -1.

Close #461

src/emulate.c

@@ -617,6 +617,8 @@ static void block_translate(riscv_t *rv, block_t *block)
             ) {
                 ir->branch_table = calloc(1, sizeof(branch_history_table_t));
                 assert(ir->branch_table);
+                memset(ir->branch_table->PC, -1,
+                       sizeof(uint32_t) * HISTORY_SIZE);
             }
             break;
         }