mod_storage_sql: Only remove old index if it exists

Avoids an error if the upgrade is performed twice..

plugins/mod_storage_sql.lua

@@ -859,16 +859,18 @@ local function upgrade_table(engine, params, apply_changes) -- luacheck: ignore
 					end
 				end
 			end)
-			if apply_changes then
-				local success = engine:transaction(function ()
-					return assert(engine:execute([[DROP INDEX "prosody_index";]]));
-				end);
-				if not success then
-					module:log("error", "Failed to delete obsolete index \"prosody_index\"");
-					return false;
+			if indices["prosody_index"] then
+				if apply_changes then
+					local success = engine:transaction(function ()
+						return assert(engine:execute([[DROP INDEX "prosody_index";]]));
+					end);
+					if not success then
+						module:log("error", "Failed to delete obsolete index \"prosody_index\"");
+						return false;
+					end
+				else
+					changes = true;
 				end
-			else
-				changes = changes or indices["prosody_index"];
 			end
 		end
 	return changes;