[stdlib] Add missing precondition check

Fixes <rdar://problem/17392712> UnsafeArray.init() should _precondition(length >= 0)

Swift SVN r19373

stdlib/core/UnsafeArray.swift.gyb

@@ -57,6 +57,7 @@
   }
   
   @public init(start: UnsafePointer<T>, length: Int) {
+    _precondition(length >= 0, "Unsafe${Mutable}Array with negative length")
     _position = start
     _end = start + length
   }