mov: Do not compute negative SAR values

This partially reverts cf70ba37ba, since
it didn't take into account when rotation is 0, but there is another
valid operation (eg. translation) in the matrix.

Found-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mov.c

@@ -35,7 +35,6 @@
 #include "libavutil/time_internal.h"
 #include "libavutil/avstring.h"
 #include "libavutil/dict.h"
-#include "libavutil/display.h"
 #include "libavutil/opt.h"
 #include "libavcodec/ac3tab.h"
 #include "avformat.h"
@@ -2579,10 +2578,9 @@ static int mov_read_tkhd(MOVContext *c, AVIOContext *pb, MOVAtom atom)
     }
 
     // transform the display width/height according to the matrix
-    // skip this if the rotation angle is 0 degrees
+    // skip this when the display matrix is the identity one
     // to keep the same scale, use [width height 1<<16]
-    if (width && height && sc->display_matrix &&
-        av_display_rotation_get(sc->display_matrix) != 0.0f) {
+    if (width && height && sc->display_matrix) {
         for (i = 0; i < 2; i++)
             disp_transform[i] =
                 (int64_t)  width  * display_matrix[0][i] +
@@ -2590,6 +2588,7 @@ static int mov_read_tkhd(MOVContext *c, AVIOContext *pb, MOVAtom atom)
                 ((int64_t) display_matrix[2][i] << 16);
 
         //sample aspect ratio is new width/height divided by old width/height
+        if (disp_transform[0] > 0 && disp_transform[1] > 0)
             st->sample_aspect_ratio = av_d2q(
                 ((double) disp_transform[0] * height) /
                 ((double) disp_transform[1] * width), INT_MAX);