avutil/log: fix zero length gnu_printf format string warning

This should fix warning reported by fate client:
http://fate.ffmpeg.org/report.cgi?time=20150917113121&slot=x86_32-linux-gnu-gcc-4.5.1-have_6regs.
Untested.
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/log.c

@@ -343,7 +343,7 @@ void av_log_default_callback(void* ptr, int level, const char* fmt, va_list vl)
 
 #if CONFIG_VALGRIND_BACKTRACE
     if (level <= BACKTRACE_LOGLEVEL)
-        VALGRIND_PRINTF_BACKTRACE("");
+        VALGRIND_PRINTF_BACKTRACE("%s", "");
 #endif
 end:
     av_bprint_finalize(part+3, NULL);