Engine: Fix ancient identical branch warning in the Polymer code

Based on commit 2aca5113, I believe the second branch was supposed to set curpicnum to overpicnum:

Here's the relevant changes from that commit:

-            pth = gltexcache((wal->overpicnum) ? wal->overpicnum : wal->picnum, wal->pal, 0);
+            if ((wal->cstat & 16) || (wal->overpicnum == 0))
+                curpicnum = wal->picnum;
+            else
+                curpicnum = wal->picnum;
+
+            if (picanm[curpicnum]&192) curpicnum += animateoffs(curpicnum,wallnum+16384);
+
+            if (!waloff[curpicnum])
+                loadtile(curpicnum);
+
+            pth = gltexcache(curpicnum, wal->pal, 0);

No idea what fixing this typo changes in effect, but if anything weird starts happening because of this, we can always revert and just remove the conditional instead.