Fixed wrong casting in `OP_LOADI32`.

Negative integer `>-65535` had wrong value, e,g, `p(-40550)` printed
`4294926746` since Nov. 2020, sigh.

src/codedump.c

@@ -172,7 +172,7 @@ codedump(mrb_state *mrb, const mrb_irep *irep)
       print_lv_a(mrb, irep, a);
       break;
     CASE(OP_LOADI32, BSS);
-      printf("OP_LOADI32\tR%d\t%d\t", a, (int)(((uint32_t)b<<16)+c));
+      printf("OP_LOADI32\tR%d\t%d\t", a, (int32_t)(((uint32_t)b<<16)+c));
       print_lv_a(mrb, irep, a);
       break;
     CASE(OP_LOADI__1, B);

src/vm.c

@@ -1134,7 +1134,7 @@ RETRY_TRY_BLOCK:
     }
 
     CASE(OP_LOADI32, BSS) {
-      SET_INT_VALUE(mrb, regs[a], (mrb_int)(((uint32_t)b<<16)+c));
+      SET_INT_VALUE(mrb, regs[a], (int32_t)(((uint32_t)b<<16)+c));
       NEXT;
     }