Fix sInv monster spellcasting logic (acrobat)

Tracers didn't have can_see_invisible set, badly breaking targeting
for spells like LRD when the player was invisible. Pretty funny,
admittedly.

crawl-ref/source/beam.cc

@@ -721,11 +721,7 @@ void bolt::initialise_fire()
     if (reflections > 0)
         nightvision = can_see_invis = true;
     else
-    {
-        // XXX: Should non-agents count as seeing invisible?
-        nightvision = agent() && agent()->nightvision();
-        can_see_invis = agent() && agent()->can_see_invisible();
-    }
+        precalc_agent_properties();
 
 #ifdef DEBUG_DIAGNOSTICS
     // Not a "real" tracer, merely a range/reachability check.
@@ -747,6 +743,15 @@ void bolt::initialise_fire()
 #endif
 }
 
+void bolt::precalc_agent_properties()
+{
+    const actor* a = agent();
+    // XXX: Should non-agents count as seeing invisible?
+    if (!a) return;
+    nightvision = a->nightvision();
+    can_see_invis = a->can_see_invisible();
+}
+
 void bolt::apply_beam_conducts()
 {
     if (!is_tracer && YOU_KILL(thrower))
@@ -2076,6 +2081,7 @@ void fire_tracer(const monster* mons, bolt &pbolt, bool explode_only,
     pbolt.source        = mons->pos();
     pbolt.source_id     = mons->mid;
     pbolt.attitude      = mons_attitude(*mons);
+    pbolt.precalc_agent_properties();
 
     // Init tracer variables.
     pbolt.foe_info.reset();

crawl-ref/source/beam.h

@@ -179,6 +179,7 @@ struct bolt
     void set_target(const dist &targ);
     void set_agent(const actor *agent);
     void setup_retrace();
+    void precalc_agent_properties();
 
     // Returns YOU_KILL or MON_KILL, depending on the source of the beam.
     killer_type  killer() const;