Prevent infinite loop

...if the map is filled with monsters, and for some reason
the drowning just won't kill you.

Infinite looping cannot currently happen, because no-one who
can drown can keep surviving the drowning once their amulet
of life saving is used up.

src/trap.c

@@ -3470,7 +3470,7 @@ drown()
 	}
 	u.uinwater = 1;
 	You("drown.");
-	for (;;) {
+	for (i = 0; i < 5; i++) { /* arbitrary number of loops */
 	    /* killer format and name are reconstructed every iteration
 	       because lifesaving resets them */
 	    pool_of_water = waterbody_name(u.ux, u.uy);