avcodec/shorten: Move buffer allocation and offset init to end of read_header()

They are time consuming operations, performing them after the other checks
improves the speed with damaged input dramatically.

Fixes: Timeout
Fixes: 2928/clusterfuzz-testcase-4992812120539136

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/shorten.c

@@ -462,12 +462,6 @@ static int read_header(ShortenContext *s)
     }
     s->nwrap = FFMAX(NWRAP, maxnlpc);
 
-    if ((ret = allocate_buffers(s)) < 0)
-        return ret;
-
-    if ((ret = init_offset(s)) < 0)
-        return ret;
-
     if (s->version > 1)
         s->lpcqoffset = V2LPCQOFFSET;
 
@@ -504,6 +498,13 @@ static int read_header(ShortenContext *s)
     }
 
 end:
+
+    if ((ret = allocate_buffers(s)) < 0)
+        return ret;
+
+    if ((ret = init_offset(s)) < 0)
+        return ret;
+
     s->cur_chan = 0;
     s->bitshift = 0;