Make execute yield the processor after every command

A player sending execute script contents quickly could theoretically
get an unfair share of the server.  Missed in commit db6fd8da
(v4.2.22), which made only the normal command loop yield.

src/lib/player/player.c

@@ -260,6 +260,7 @@ execute(void)
 	    failed = 1;
 	} else if (dispatch(buf, NULL) < 0)
 	    failed = 1;
+	empth_yield();
     }
     if (failed) {
 	while (recvclient(buf, sizeof(buf)) >= 0) ;