bignum.c: avoid use of uninitialized value in Integer.sqrt

This is a follow-up fix to r57713. estimate_initial_sqrt() didn't
initialize BDIGITs except the topmost two, letting Integer.sqrt return
wrong result in the fast path, such as for (1<<504).

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60611 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

bignum.c

@@ -6830,6 +6830,7 @@ estimate_initial_sqrt(VALUE *xp, const size_t xn, const BDIGIT *nds, size_t len)
     if (rshift >= 0) {
 	d <<= rshift;
     }
+    BDIGITS_ZERO(xds, xn-2);
     bdigitdbl2bary(&xds[xn-2], 2, d);
 
     if (!lowbits) return NULL; /* special case, exact result */