bignum.c (estimate_initial_sqrt): prevent integer overflow

`Integer.sqrt(0xffff_ffff_ffff_ffff ** 2)` caused assertion failure
because of integer overflow.  [ruby-core:95453] [Bug #16269]

bignum.c

@@ -6888,7 +6888,15 @@ estimate_initial_sqrt(VALUE *xp, const size_t xn, const BDIGIT *nds, size_t len)
     rshift /= 2;
     rshift += (2-(len&1))*BITSPERDIG/2;
     if (rshift >= 0) {
-	d <<= rshift;
+        if (nlz((BDIGIT)d) + rshift >= BITSPERDIG) {
+            /* (d << rshift) does cause overflow.
+             * example: Integer.sqrt(0xffff_ffff_ffff_ffff ** 2)
+             */
+            d = ~(BDIGIT_DBL)0;
+        }
+        else {
+            d <<= rshift;
+        }
     }
     BDIGITS_ZERO(xds, xn-2);
     bdigitdbl2bary(&xds[xn-2], 2, d);

test/ruby/test_integer.rb

@@ -640,6 +640,9 @@ class TestInteger < Test::Unit::TestCase
       failures << n  unless root*root <= n && (root+1)*(root+1) > n
     end
     assert_empty(failures, bug13440)
+
+    x = 0xffff_ffff_ffff_ffff
+    assert_equal(x, Integer.sqrt(x ** 2), "[ruby-core:95453]")
   end
 
   def test_fdiv