std.math.big.int: fix shiftRight sign handling

Closes ziglang#21311

The sign of the result `r` needs to be initialized before the correction
`r.addScalar(r.toConst(), -1)`, or the intended end result could be off
by 2 (depending on the original sign of `r`).

lib/std/math/big/int.zig

@@ -1202,6 +1202,7 @@ pub const Mutable = struct {
         llshr(r.limbs[0..], a.limbs[0..a.limbs.len], shift);
 
         r.len = a.limbs.len - full_limbs_shifted_out;
+        r.positive = a.positive;
         if (nonzero_negative_shiftout) {
             if (full_limbs_shifted_out > 0) {
                 r.limbs[a.limbs.len - full_limbs_shifted_out] = 0;
@@ -1210,7 +1211,6 @@ pub const Mutable = struct {
             r.addScalar(r.toConst(), -1);
         }
         r.normalize(r.len);
-        r.positive = a.positive;
     }
 
     /// r = ~a under 2s complement wrapping semantics.

lib/std/math/big/int_test.zig

@@ -2083,6 +2083,15 @@ test "shift-right negative" {
     try a.shiftRight(&a, 1);
     a.setSign(true);
     try testing.expect(try a.to(u64) == 0x8000000000000000);
+
+    var arg7 = try Managed.initSet(testing.allocator, -32767);
+    defer arg7.deinit();
+    a.setSign(false);
+    try a.shiftRight(&arg7, 4);
+    try testing.expect(try a.to(i16) == -2048);
+    a.setSign(true);
+    try a.shiftRight(&arg7, 4);
+    try testing.expect(try a.to(i16) == -2048);
 }
 
 test "sat shift-left simple unsigned" {