deshake: variable used uninitialized

Sometimes the scan finds nothing that qualifies for addition to
the array and pos is zero after the loops.  The code forces pos to
1 and the array is then processed as if it had one valid element in it,
producing some amusing but not very useful results.

I don't see the rationale for this.  If pos is zero coming out of the
loops, the only appropriate thing to do is set t->angle to zero.  The
attached patch does that.  It's worked properly in several tests so far.

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

libavfilter/vf_deshake.c

@@ -292,14 +292,15 @@ static void find_motion(DeshakeContext *deshake, uint8_t *src1, uint8_t *src2,
         }
     }
 
-    pos = FFMAX(1, pos);
-
-    center_x /= pos;
-    center_y /= pos;
-
-    t->angle = clean_mean(angles, pos);
-    if (t->angle < 0.001)
-        t->angle = 0;
+    if (pos) {
+         center_x /= pos;
+         center_y /= pos;
+         t->angle = clean_mean(angles, pos);
+         if (t->angle < 0.001)
+              t->angle = 0;
+    } else {
+         t->angle = 0;
+    }
 
     // Find the most common motion vector in the frame and use it as the gmv
     for (y = deshake->ry * 2; y >= 0; y--) {