simplify _getCString

Author: Azoy

stdlib/public/core/StringStorage.swift

@@ -76,9 +76,8 @@ extension _AbstractStringStorage {
       fallthrough
     case (_cocoaUTF8Encoding, _):
       guard maxLength >= count + 1 else { return 0 }
-      let buffer = UnsafeMutableBufferPointer(start: outputPtr, count: maxLength)
-      _ = buffer.initialize(from: UnsafeBufferPointer(start: start, count: count))
-      buffer[count] = 0
+      outputPtr.initialize(from: start, count: count)
+      outputPtr[count] = 0
       return 1
     default:
       return  _cocoaGetCStringTrampoline(self, outputPtr, maxLength, encoding)