Fix a build error

Author: tannergooding

src/libraries/System.Private.CoreLib/src/System/Number.Dragon4.cs

@@ -154,6 +154,10 @@ private static uint Dragon4(ulong mantissa, int exponent, uint mantissaHighBitId
                     // scaledMarginLow = 2 * 2^(-1)
                     BigInteger.SetUInt32(out scaledMarginLow, 1);
                 }
+
+                // This is unused for this path, but we need it viewed as "initialized" so the
+                // scaledMarginHigh tracking works as expected.
+                Unsafe.SkipInit(out optionalMarginHigh);
             }
 
             scoped ref BigInteger scaledMarginHigh = ref (hasUnequalMargins ? ref optionalMarginHigh : ref scaledMarginLow);