return legal indexes for some display.h macros

Some static analyzers flagged the last-resort values as
out of bounds (which they were).

There's a small number of other complaint-suppression items in here too,
but nothing drastic.

src/apply.c

@@ -3161,13 +3161,12 @@ static boolean
 find_poleable_mon(coord *pos, int min_range, int max_range)
 {
     struct monst *mtmp;
-    coord mpos;
+    coord mpos = { 0, 0 }; /* no candidate location yet */
     boolean impaired;
     coordxy x, y, lo_x, hi_x, lo_y, hi_y, rt;
     int glyph;
 
     impaired = (Confusion || Stunned || Hallucination);
-    mpos.x = mpos.y = 0; /* no candidate location yet */
     rt = isqrt(max_range);
     lo_x = max(u.ux - rt, 1), hi_x = min(u.ux + rt, COLNO - 1);
     lo_y = max(u.uy - rt, 0), hi_y = min(u.uy + rt, ROWNO - 1);