Fix: force steed dismount if steed becomes hostile from the player wielding or wearing a bane artifact vs the steed.

Players aren't supposed to be able to stay on their steed if the steed
becomes hostile. This caused a serious issue with someones game earlier
today - they were riding a tame dragon, they put on dragonbane which
made their steed hostile, but was still mounted. Went to use the travel
command, and upon traveling, 'placing steed onto map, mstate:0, on
Dlvl:16 ?' error occurred every tile they moved to. The steed
escaped upstairs, but still thought the player was mounted. Game could
not save properly and became corrupted, which forced a restore from an
earlier backup.

Thankfully an easy fix, but I'm surprised this behavior (forcing steed
dismount) wasn't thought of from the get-go.

Author: k21971

doc/evilhack-changelog.md

@@ -3449,4 +3449,6 @@ The following changes to date are:
   whilst using thievery skill
 - Fix: Draugr trying to read a spellbook and the spellbook is
   destroyed while reading it
+- Fix: force steed dismount if steed becomes hostile from the player
+  wielding or wearing a bane artifact vs the steed
 

src/allmain.c

@@ -630,6 +630,8 @@ boolean resuming;
                             } else if (mtmp->mpeaceful || mtmp->mtame) {
                                 setmangry(mtmp, FALSE);
                                 mtmp->mpeaceful = mtmp->mtame = 0;
+                                if (u.usteed)
+                                    dismount_steed(DISMOUNT_THROWN);
                             }
                         }
                     }