avfilter/phase_template: Fix left-shift of negative numbers

Affected the filter-phase FATE-test.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/phase_template.c

@@ -46,7 +46,7 @@
  *
  * (The result is actually multiplied by 25)
  */
-#define DIFF(a, as, b, bs) ((t) = ((*(a) - (b)[bs]) << 2) + (a)[(as) << 1] - (b)[-(bs)], (t) * (t))
+#define DIFF(a, as, b, bs) ((t) = ((*(a) - (b)[bs]) * 4) + (a)[(as) * 2] - (b)[-(bs)], (t) * (t))
 
 /*
  * Find which field combination has the smallest average squared difference