fix: scope variable for map field to avoid redeclaration (protobufjs#1717) (protobufjs#1718)

Co-authored-by: Alexander Fenster <fenster@google.com>

Author: steinybot

src/decoder.js

@@ -33,7 +33,7 @@ function decoder(mtype) {
         var field = mtype._fieldsArray[i].resolve(),
             type  = field.resolvedType instanceof Enum ? "int32" : field.type,
             ref   = "m" + util.safeProp(field.name); gen
-            ("case %i:", field.id);
+            ("case %i: {", field.id);
 
         // Map fields
         if (field.map) { gen
@@ -104,8 +104,9 @@ function decoder(mtype) {
         else gen
                 ("%s=r.%s()", ref, type);
         gen
-                ("break");
-    // Unknown fields
+                ("break")
+            ("}");
+        // Unknown fields
     } gen
             ("default:")
                 ("r.skipType(t&7)")