[stdlib] improve UnsafeMutableRawBufferPointer.copyMemory

- The previous implementation trapped when the source buffer was empty. That behaviour is both not documented and unnecessary. If the source buffer is empty, its length is necessarily shorter or equal than the length of the destination.
- The updated version simply returns when the source buffer is empty.

Author: glessard

stdlib/public/core/UnsafeRawBufferPointer.swift.gyb

@@ -438,7 +438,9 @@ extension Unsafe${Mutable}RawBufferPointer {
   public func copyMemory(from source: UnsafeRawBufferPointer) {
     _debugPrecondition(source.count <= self.count,
       "${Self}.copyMemory source has too many elements")
-    baseAddress?.copyMemory(from: source.baseAddress!, byteCount: source.count)
+    if let baseAddress = baseAddress, let sourceAddress = source.baseAddress {
+      baseAddress.copyMemory(from: sourceAddress, byteCount: source.count)
+    }
   }
 
   /// Copies from a collection of `UInt8` into this buffer's memory.