internal/impl: fix non-atomic access of MessageState.mi

MessageState's mi field must only be accessed via atomic operations.
Rename the field to 'atomicMessageInfo' to make it incorrect access
obvious. Fix one incorrect non-atomic access.

Change-Id: If80343fb1b82186416f007ca5b340a4926b1cd3c
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/221419
Reviewed-by: Joe Tsai <joetsai@google.com>
diff --git a/internal/impl/message_reflect.go b/internal/impl/message_reflect.go
index f5f7f2b..aac55ee 100644
--- a/internal/impl/message_reflect.go
+++ b/internal/impl/message_reflect.go
@@ -256,7 +256,7 @@
 	pragma.DoNotCompare
 	pragma.DoNotCopy
 
-	mi *MessageInfo
+	atomicMessageInfo *MessageInfo
 }
 
 type messageState MessageState