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/hack.c

@@ -1363,7 +1363,7 @@ domove()
 
         if (boulder)
             Strcpy(buf, ansimpleoname(boulder));
-        else if (solid)
+        else if (solid && glyph_is_cmap(glyph))
             Strcpy(buf, the(defsyms[glyph_to_cmap(glyph)].explanation));
         else if (!Underwater)
             Strcpy(buf, "thin air");