While building gnash for coverity, I saw this warning:
    
  CXX      DefineBitsTag.lo
swf/DefineBitsTag.cpp: In function 'gnash::FileType gnash::SWF::{anonymous}::c\
heckFileType(gna
sh::SWFStream&)':
swf/DefineBitsTag.cpp:301:18: warning: unused variable 'start' [-Wunused-varia\
ble]
     const size_t start = in.tell();
                  ^
This fix should fix the problem:

diff --git a/libcore/swf/DefineBitsTag.cpp b/libcore/swf/DefineBitsTag.cpp
index a7c8b79..9967921 100644
--- a/libcore/swf/DefineBitsTag.cpp
+++ b/libcore/swf/DefineBitsTag.cpp
@@ -297,10 +297,6 @@ readDefineBitsJpeg(SWFStream& /*in*/, movie_definition& m\
)
 FileType
 checkFileType(SWFStream& in)
 {
-#ifndef NDEBUG
-    const size_t start = in.tell();
-#endif
-
     const size_t bytes = 3;
     char buf[bytes];

--
Happy hacking
Petter Reinholdtsen