no leash-related message is given when leashed pet yellow light explodes

mondead() -> m_detach() -> m_unleash() suppresses
the m_unleash() slack message, so deliver it in
the caller explmm() in those circumstances.

(The issue of whether it should be possible to leash light
is side-stepped.)

bug H7406, 1548

doc/fixes36.2

@@ -139,6 +139,7 @@ drum of earthquake was causing deafness but oddly enough only when used up
 known bear trap was being forgotten about by a player polymorphed into a
 	flying monster if the player unsuccessfully tried to #untrap it and
 	moved onto the trap square as a result
+no leash-related message is given when a leashed pet yellow light explodes
 
 
 Fixes to Post-3.6.1 Problems that Were Exposed Via git Repository

src/mhitm.c

@@ -781,10 +781,17 @@ struct attack *mattk;
 
     /* Kill off aggressor if it didn't die. */
     if (!(result & MM_AGR_DIED)) {
+        boolean was_leashed = (magr->mleashed);
+
         mondead(magr);
         if (!DEADMONSTER(magr))
             return result; /* life saved */
         result |= MM_AGR_DIED;
+        
+        /* mondead() -> m_detach() -> m_unleash() always suppresses
+           the m_unleash() slack message, so deliver it here instead */
+        if (was_leashed)
+            Your("leash falls slack.");
     }
     if (magr->mtame) /* give this one even if it was visible */
         You(brief_feeling, "melancholy");