prevent a couple of reported array index segfaults

Changes to be committed:
	modified:   src/hack.c
	modified:   src/pager.c

Don't use glyph_to_cmap as an array index into
the defsyms[] array unless it really is a cmap.

Recent situation: glyph_to_cmap will return
NO_GLYPH for the unknown monster glyph 'I', which
is not a valid index for the defsyms[] array.

src/pager.c

@@ -273,7 +273,8 @@ lookat(x, y, buf, monbuf)
 	Strcpy(buf, Is_airlevel(&u.uz) ? "cloudy area" : "fog/vapor cloud");
 	break;
     default:
-	Strcpy(buf,defsyms[glyph_to_cmap(glyph)].explanation);
+	if (glyph_is_cmap(glyph))
+	    Strcpy(buf,defsyms[glyph_to_cmap(glyph)].explanation);
 	break;
     }