Fix an assert() in os_unix.c that is not true when using RBU.

FossilOrigin-Name: 028ed8618af012b51104a06cc155b45d98bc4ff833d4431b9c38fd6851a74e82

src/os_unix.c

@@ -4879,7 +4879,9 @@ static void unixShmBarrier(
 ){
   UNUSED_PARAMETER(fd);
   sqlite3MemoryBarrier();         /* compiler-defined memory barrier */
-  assert( unixFileMutexNotheld((unixFile*)fd) );
+  assert( fd->pMethods->xLock==nolockLock 
+       || unixFileMutexNotheld((unixFile*)fd) 
+  );
   unixEnterMutex();               /* Also mutex, for redundancy */
   unixLeaveMutex();
 }