impossible fix - potentially invalid feedback

From a bug report, the message displayed to the screen by
impossible() might be different from the one written into paniclog, if it
had argument subsitution/formatting.  I couldn't reproduce that myself,
but stdarg.h/varargs.h is tricky stuff and I think that passing the va_list
to a routine which steps through it requires that va_start be called again
if you're going to use the va_list a second time.  This changes impossible()
to handle its arguments only once, like panic().

doc/fixes35.0

@@ -847,6 +847,7 @@ when a monster zapped by polymorph drops inventory because of its new form,
 entering an untended shop while blind gave an inappropriate message
 engraving feedback about partial text when weapon became too dull to finish
 	was lacking sentence-ending period
+impossible() might display inaccurate feedback after updating paniclog
 
 
 Platform- and/or Interface-Specific Fixes

src/pline.c

@@ -302,17 +302,17 @@ raw_printf VA_DECL(const char *, line)
 /*VARARGS1*/
 void
 impossible VA_DECL(const char *, s)
+	char pbuf[2*BUFSZ];
 	VA_START(s);
 	VA_INIT(s, const char *);
 	if (program_state.in_impossible)
-		panic("impossible called impossible");
+	    panic("impossible called impossible");
+
 	program_state.in_impossible = 1;
-	{
+	Vsprintf(pbuf, s, VA_ARGS);
-	    char pbuf[BUFSZ];
+	pbuf[BUFSZ-1] = '\0';		/* sanity */
-	    Vsprintf(pbuf,s,VA_ARGS);
+	paniclog("impossible", pbuf);
-	    paniclog("impossible", pbuf);
+	pline("%s", pbuf);
-	}
-	vpline(s,VA_ARGS);
 	pline("Program in disorder - perhaps you'd better #quit.");
 	program_state.in_impossible = 0;
 	VA_END();