db/proxy: merge redundant string::push_back() calls

src/db/UpnpDatabasePlugin.cxx

@@ -254,13 +254,11 @@ dquote(std::string &out, const char *in)
 		case '\\':
 		case '"':
 			out.push_back('\\');
-			out.push_back(*in);
 			break;
+		}
 
-		default:
 		out.push_back(*in);
 	}
-	}
 
 	out.push_back('"');
 }