mrb_str_concat() may call VM resursively thus may reallocate VM stack; close #3000

src/vm.c

@@ -2178,6 +2178,7 @@ RETRY_TRY_BLOCK:
     CASE(OP_STRCAT) {
       /* A B    R(A).concat(R(B)) */
       mrb_str_concat(mrb, regs[GETARG_A(i)], regs[GETARG_B(i)]);
+      regs = mrb->c->stack;
       NEXT;
     }