Destroy water walking boots on lava first.

This avoids a player being left standing on lava if their boots are burned up and
they survive the damage.

This change is originally due to Steve Melenchuk, fixed up by Derrick Sund.

src/trap.c

@@ -4708,14 +4708,24 @@ lava_effects()
 		    !obj_resists(obj, 0, 0))	/* for invocation items */
 		obj->in_use = TRUE;
 
+    /* Check whether we should burn away boots *first* so we know whether to
+     * make the player sink into the lava. Assumption: water walking only comes
+     * from boots. */
+    if (Wwalking && uarmf && is_organic(uarmf) && !uarmf->oerodeproof) {
+        obj = uarmf;
+        pline("Your %s into flame!", aobjnam(obj, "burst"));
+        setequip(os_armf, NULL, em_silent);
+        useupall(obj);
+    }
+
     if (!Fire_resistance) {
-	if(Wwalking) {
+        if(Wwalking) {
 	    pline_The("lava here burns you!");
 	    if (usurvive) {
 		losehp(dmg, lava_killer, KILLED_BY);	/* lava damage */
 		goto burn_stuff;
 	    }
-	} else
+        } else
 	    You("fall into the lava!");
 
 	usurvive = Lifesaved || discover;