ppc/mpegvideo_altivec: Change assert to av_assert2()

And move it to where the variable is used
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ppc/mpegvideo_altivec.c

@@ -42,8 +42,6 @@ static void dct_unquantize_h263_altivec(MpegEncContext *s,
     int i, level, qmul, qadd;
     int nCoeffs;
 
-    assert(s->block_last_index[n]>=0);
-
     qadd = (qscale - 1) | 1;
     qmul = qscale << 1;
 
@@ -59,6 +57,7 @@ static void dct_unquantize_h263_altivec(MpegEncContext *s,
         nCoeffs= 63; //does not always use zigzag table
     } else {
         i = 0;
+        av_assert2(s->block_last_index[n]>=0);
         nCoeffs= s->intra_scantable.raster_end[ s->block_last_index[n] ];
     }