avcodec/mobiclip: Rewrite code to make it clearer

In order to know that the earlier code did not use uninitialized
values one needs to know that the lowest four bits of each used
value of pframe_block4x4_coefficients_tab do not vanish identically.
E.g. Coverity did not get this and warned about it in ticket #1466632.
Fix this by slightly rewriting the code.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/mobiclip.c

@@ -491,7 +491,7 @@ static int add_pframe_coefficients(AVCodecContext *avctx, AVFrame *frame,
     int ret, idx = get_ue_golomb_31(gb);
 
     if (idx == 0) {
-        ret = add_coefficients(avctx, frame, bx, by, size, plane);
+        return add_coefficients(avctx, frame, bx, by, size, plane);
     } else if ((unsigned)idx < FF_ARRAY_ELEMS(pframe_block4x4_coefficients_tab)) {
         int flags = pframe_block4x4_coefficients_tab[idx];
 
@@ -505,11 +505,10 @@ static int add_pframe_coefficients(AVCodecContext *avctx, AVFrame *frame,
                 flags >>= 1;
             }
         }
+        return 0;
     } else {
-        ret = AVERROR_INVALIDDATA;
+        return AVERROR_INVALIDDATA;
     }
-
-    return ret;
 }
 
 static int adjust(int x, int size)