Add comment on incrementing p_mt_info->num_workers

In av1_create_workers(), it is important to increment
p_mt_info->num_workers in the for loop that creates the worker threads,
so that av1_terminate_workers() still works after a partial failure.
Add a comment to explain this.

Change-Id: I55c7fa341d32b577c89104733cbe8da78a8a0d76

av1/encoder/ethread.c

diff --git a/av1/encoder/ethread.c b/av1/encoder/ethread.c
index 636a418..0422856 100644
--- a/av1/encoder/ethread.c
+++ b/av1/encoder/ethread.c
@@ -1117,6 +1117,8 @@
     }
     winterface->sync(worker);
 
+    // Ensure p_mt_info->num_workers is the number of threads successfuly
+    // created. Important to av1_terminate_workers().
     ++p_mt_info->num_workers;
   }
 }