Make MojoWaitMany() accept zero handles.

mojo/public/c/system/wait.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file contains functions for waiting on handles (directly).
 //
 // Note: This header should be compilable as C.
 
 #ifndef MOJO_PUBLIC_C_SYSTEM_WAIT_H_
 #define MOJO_PUBLIC_C_SYSTEM_WAIT_H_
@@ skipping 53 matching lines @@
 //   - It becomes known that no signal in some |signals[i]| will ever be
 //     satisfied.
 //   - |deadline| has passed.
 //
 // This means that |MojoWaitMany()| behaves as if |MojoWait()| were called on
 // each handle/signals pair simultaneously, completing when the first
 // |MojoWait()| would complete.
 //
 // See |MojoWait()| for more details about |deadline|.
 //
+// Note that if |num_handles| is 0, this will wait until |deadline| has passed
+// (possibly forever if it is |MOJO_DEADLINE_INDEFINITE|), and handles|,
+// |signals|, |result_index|, and |signals_states| will all be ignored (so they
+// may be null or invalid pointers).
+//
 // |result_index| (optional) is used to return the index of the handle that
 //     caused the call to return. For example, the index |i| (from 0 to
 //     |num_handles-1|) if |handle[i]| satisfies a signal from |signals[i]|. You
 //     must manually initialize this to a suitable sentinel value (e.g. -1)
 //     before you make this call because this value is not updated if there is
 //     no specific handle that causes the function to return. Pass null if you
 //     don't need this value to be returned.
 //
 // |signals_states| (optional) points to an array of size |num_handles| of
 //     MojoHandleSignalsState. See |MojoHandleSignalsState| for more details
@@ skipping 22 matching lines @@
                         const MojoHandleSignals* MOJO_RESTRICT signals,  // In.
                         uint32_t num_handles,                            // In.
                         MojoDeadline deadline,                           // In.
                         uint32_t* MOJO_RESTRICT result_index,  // Optional out.
                         struct MojoHandleSignalsState* MOJO_RESTRICT
                             signals_states);  // Optional out.
 
 MOJO_END_EXTERN_C
 
 #endif  // MOJO_PUBLIC_C_SYSTEM_WAIT_H_