Protect against null shopkeeper in shk_other_services. The fuzzer fou…

…nd a shopkeeper that didn't have the shopkeeper struct for some reason when #chat was used.

src/shk.c

@@ -1786,6 +1786,9 @@ shk_other_services()
 
     /* Init the shopkeeper */
     shkp = shop_keeper(/* roomno= */*u.ushops);
+    if (!shkp)
+        return result;
+
     shkdat = &mons[ERAC(shkp)->rmnum];
 
     /* Init your name */