vp8_peek_si_internal: quiet static analysis warning

the vpx_decoder layer guarantees that when called directly this won't
receive NULL data and the reuse via decode() is protected by a NULL data
check and 0 size check (NULL data and non-zero data size is protected by
the vpx_decoder layer).

Change-Id: I7437fb5ca4e4aa431963d55b909d4d920f339be3

vp8/vp8_dx_iface.c

@@ -9,6 +9,7 @@
  */
 
 
+#include <assert.h>
 #include <stdlib.h>
 #include <string.h>
 #include "./vp8_rtcd.h"
@@ -154,6 +155,8 @@ static vpx_codec_err_t vp8_peek_si_internal(const uint8_t *data,
 {
     vpx_codec_err_t res = VPX_CODEC_OK;
 
+    assert(data != NULL);
+
     if(data + data_sz <= data)
     {
         res = VPX_CODEC_INVALID_PARAM;