Avoid temporary byte[] allocation in Pkcs' HeapBlockReatiner.AllocAsciiString (#75136)

Author: stephentoub

src/libraries/System.Security.Cryptography.Pkcs/src/Internal/Cryptography/Pal/Windows/HeapBlockRetainer.cs

@@ -42,15 +42,17 @@ public IntPtr Alloc(int howMany, int cbElement)
             return Alloc(cbSize);
         }
 
-        public IntPtr AllocAsciiString(string s)
+        public unsafe IntPtr AllocAsciiString(string s)
         {
-            byte[] b = Encoding.ASCII.GetBytes(s);
-            IntPtr pb = Alloc(b.Length + 1);
-            Marshal.Copy(b, 0, pb, b.Length);
-            unsafe
-            {
-                ((byte*)pb)[b.Length] = 0; // NUL termination.
-            }
+            int length = Encoding.ASCII.GetByteCount(s);
+            length++; // for null termination
+
+            IntPtr pb = Alloc(length);
+
+            var ascii = new Span<byte>((byte*)pb, length);
+            Encoding.ASCII.GetBytes(s, ascii);
+            ascii[^1] = 0;
+
             return pb;
         }