fix crash when using ';' to look at I monsters

Reported to the mailing list.  If you set your monster symbol options, and
use ';' and select a known invisible monster by screen, a crash would occur
accessing a null pointer.

doc/fixes34.1

@@ -118,6 +118,7 @@ prevent "remove_object: obj not on floor" panic for iron ball placement if
 	riding while punished leads to a fall off steed when changing levels
 specifying -D (or -X) to enter explore mode while restarting from a save
 	file was lost in the restore process
+fix crash when using lookat on an known invisible monster with monster syms set
 
 
 Platform- and/or Interface-Specific Fixes

src/pager.c

@@ -531,7 +531,8 @@ do_look(quick)
 
 	/* Check for monsters */
 	for (i = 0; i < MAXMCLASSES; i++) {
-	    if (sym == (from_screen ? monsyms[i] : def_monsyms[i])) {
+	    if (sym == (from_screen ? monsyms[i] : def_monsyms[i]) &&
+		monexplain[i]) {
 		need_to_look = TRUE;
 		if (!found) {
 		    Sprintf(out_str, "%c       %s", sym, an(monexplain[i]));