[stdlib] U[R]BP: Restore compile-time condition lost with removed precondition

Author: lorentey

stdlib/public/core/UnsafeBufferPointer.swift.gyb

@@ -414,7 +414,7 @@ extension Unsafe${Mutable}BufferPointer {
       count == 0 || start != nil,
       "Unsafe${Mutable}BufferPointer has a nil start and nonzero count")
     _position = start
-    self.count = count
+    self.count = _assumeNonNegative(count)
   }
 
   @inlinable // unsafe-performance

stdlib/public/core/UnsafeRawBufferPointer.swift.gyb

@@ -436,7 +436,7 @@ extension Unsafe${Mutable}RawBufferPointer {
     _debugPrecondition(count == 0 || start != nil,
       "${Self} has a nil start and nonzero count")
     _position = start
-    _end = start.map { $0 + count }
+    _end = start.map { $0 + _assumeNonNegative(count) }
   }
 
   /// Creates a new buffer over the same memory as the given buffer.