mxg: fix compiler warning for uninitialized variables

Based on work by: Jean First <jeanfirst@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mxg.c

@@ -104,7 +104,7 @@ static int mxg_update_cache(AVFormatContext *s, unsigned int cache_size)
     /* reallocate internal buffer */
     if (current_pos > current_pos + cache_size)
         return AVERROR(ENOMEM);
-    if (mxg->soi_ptr) soi_pos = mxg->soi_ptr - mxg->buffer;
+    soi_pos = mxg->soi_ptr - mxg->buffer;
     mxg->buffer = av_fast_realloc(mxg->buffer, &mxg->buffer_size,
                                   current_pos + cache_size +
                                   FF_INPUT_BUFFER_PADDING_SIZE);