Escape from controlled polymorph prompt no longer causes random poly

This was one of the remaining "a single typo causes a major gameplay
disruption" behaviors (i.e. turning into something large and breaking
armor, or into a dragon). Since this prompt only appears when the
polymorph is controlled, there's no reason to punish this in particular,
so I changed it to instead inform the player how they can explicitly
request a random polymorph, then prompt again.

However, I did not touch the "That's enough tries! Have a random
polymorph!" behavior if the loop is triggered enough times (currently
5).

doc/xnh-changelog-9.0.md

@@ -67,6 +67,10 @@ changes:
 - When polymorphed into a monster with an explosion attack, you can #monster to
   intentionally explode. (This was already possible by attacking a monster or
   force-fighting thin air.)
+- Escaping out of a polymorph prompt no longer causes a random polymorph; it
+  instead gives instructions on how to explicitly request that and prompts
+  again. (However, escaping or failing to enter a valid form 5 times still
+  causes a random polymorph.)
 
 ### Architectural changes
 

src/polyself.c

@@ -499,7 +499,8 @@ polyself(int psflags)
                     pline1(Never_mind);
                     return;
                 }
-                Strcpy(buf, "*"); /* resort to random */
+                pline("Enter \"random\" or \"*\" for a random polymorph.");
+                continue;
             }
             if (!strcmp(buf, "*") || !strcmp(buf, "random")) {
                 /* explicitly requesting random result */