stdlib/Array: fix an issue in fast enumeration

Array did not initialize fast enumeration state if it was empty.
Surprisingly, this did not break code that is generated by Clang
currently.  (But as far as I understand fast enumeration, it may abort
the program because mutation pointer is null.)

Swift SVN r21940

stdlib/core/ContiguousArrayBuffer.swift

@@ -101,7 +101,8 @@ final internal class _ContiguousArrayStorage<T> : _NSSwiftArray {
       return batchCount
     }
     else {
-      enumerationState.state = UInt(min(count, 1))
+      enumerationState.state = UInt(max(count, 1))
+      state.memory = enumerationState
       return 0
     }
   }