vf_unsharp: Fix false ‘src2’ may be used uninitialized in this function warning.

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

libavfilter/vf_unsharp.c

@@ -73,7 +73,7 @@ static void apply_unsharp(      uint8_t *dst, int dst_stride,
 
     int32_t res;
     int x, y, z;
-    const uint8_t *src2;
+    const uint8_t *src2 = NULL;  //silence a warning
 
     if (!fp->amount) {
         if (dst_stride == src_stride)