Fix assertion

Author: kzrnm

src/libraries/System.Runtime.Numerics/src/System/Numerics/BigIntegerCalculator.SquMul.cs

@@ -160,7 +160,8 @@ internal const
         public static void Multiply(ReadOnlySpan<uint> left, ReadOnlySpan<uint> right, Span<uint> bits)
         {
             Debug.Assert(left.Length >= right.Length);
-            Debug.Assert(bits.Length == left.Length + right.Length);
+            Debug.Assert(bits.Length >= left.Length + right.Length);
+            Debug.Assert(!bits.ContainsAnyExcept(0u));
 
             if (left.Length - right.Length < 3)
             {
@@ -175,7 +176,8 @@ public static void Multiply(ReadOnlySpan<uint> left, ReadOnlySpan<uint> right, S
         private static void MultiplyFarLength(ReadOnlySpan<uint> left, ReadOnlySpan<uint> right, Span<uint> bits)
         {
             Debug.Assert(left.Length - right.Length >= 3);
-            Debug.Assert(bits.Length == left.Length + right.Length);
+            Debug.Assert(bits.Length >= left.Length + right.Length);
+            Debug.Assert(!bits.ContainsAnyExcept(0u));
 
             // Executes different algorithms for computing z = a * b
             // based on the actual length of b. If b is "small" enough
@@ -370,7 +372,8 @@ stackalloc uint[StackAllocThreshold]
         private static void MultiplyNearLength(ReadOnlySpan<uint> left, ReadOnlySpan<uint> right, Span<uint> bits)
         {
             Debug.Assert(left.Length - right.Length < 3);
-            Debug.Assert(bits.Length == left.Length + right.Length);
+            Debug.Assert(bits.Length >= left.Length + right.Length);
+            Debug.Assert(!bits.ContainsAnyExcept(0u));
 
             // Executes different algorithms for computing z = a * b
             // based on the actual length of b. If b is "small" enough