fix obscure secret corridor bug

magic_map_background() would overwrite remembered objects and traps.
That meant discovery of secret corridors by secret door detection
or ^E would forget embedded objects at their locations.

doc/fixes3-7-0.txt

@@ -1460,6 +1460,10 @@ region expiration could report "the gas cloud around you dissipates" and also
 	"you see a gas cloud dissipate" for the same cloud spot
 reduce shopkeeper's innate speed from 18 to 16 so that a hasted shopkeeper
 	doesn't always get 2 moves per turn
+when a secret corridor was discovered by wand of secret door detection or by
+	wizard mode ^E and converted into a regular corridor, if there was a
+	formerly embbered object at the spot, presence of the object would be
+	forgotten unless within range of a light source
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository

src/display.c

@@ -248,7 +248,7 @@ magic_map_background(coordxy x, coordxy y, int show)
         else if (lev->typ == CORR && glyph == cmap_to_glyph(S_litcorr))
             glyph = cmap_to_glyph(S_corr);
     }
-    if (svl.level.flags.hero_memory)
+    if (svl.level.flags.hero_memory && glyph_is_cmap(lev->glyph))
         lev->glyph = glyph;
     if (show)
         show_glyph(x, y, glyph);
@@ -1120,6 +1120,8 @@ tether_glyph(coordxy x, coordxy y)
  *
  * DISP_BEAM   - Display the given glyph at each location, but do not erase
  *               any until the close call.
+ * DISP_ALL    - Same as DISP_BEAM except glyph is shown at the specified
+ *               spot even when that spot can't be seen.
  * DISP_TETHER - Display a tether glyph at each location, and the tethered
  *               object at the farthest location, but do not erase any
  *               until the return trip or close.
@@ -1277,7 +1279,7 @@ flash_glyph_at(coordxy x, coordxy y, int tg, int rpt)
     rpt *= 2; /* two loop iterations per 'count' */
     glyph[0] = tg;
     glyph[1] = (svl.level.flags.hero_memory) ? levl[x][y].glyph
-                                         : back_to_glyph(x, y);
+                                             : back_to_glyph(x, y);
     /* even iteration count (guaranteed) ends with glyph[1] showing;
        caller might want to override that, but no newsym() calls here
        in case caller has tinkered with location visibility */
@@ -1974,13 +1976,14 @@ show_glyph(coordxy x, coordxy y, int glyph)
         && !program_state.in_getlev
         && (oldglyph != glyph || gg.gbuf[y][x].gnew)) {
         int c = glyph_to_cmap(glyph);
+
         if ((glyph_is_nothing(oldglyph) || glyph_is_unexplored(oldglyph)
              || is_cmap_furniture(c))
             && !is_cmap_wall(c) && !is_cmap_room(c)) {
             if ((a11y.mon_notices && glyph_is_monster(glyph))
                 || (glyph_is_monster(oldglyph))
                 || u_at(x, y)) {
-                /* nothing */
+                ; /* nothing */
             } else {
                 show_glyph_change = TRUE;
             }