Use os/2 palette even if it contains less than 256 entries.

Fixes 11Bios13.bmp.

libavcodec/bmp.c

@@ -248,6 +248,8 @@ static int bmp_decode_frame(AVCodecContext *avctx,
             } else if (t) {
                 colors = t;
             }
+        } else {
+            colors = FFMIN(256, (hsize-ihsize-14) / 3);
         }
         buf = buf0 + 14 + ihsize; //palette location
         // OS/2 bitmap, 3 bytes per palette entry