segfault fix

thanks to takis for finding the cause of this


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@6325 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavcodec/eval.c

@@ -87,6 +87,7 @@ static double evalPrimary(Parser *p){
     p->s= strchr(p->s, '(');
     if(p->s==NULL){
         av_log(NULL, AV_LOG_ERROR, "Parser: missing ( in \"%s\"\n", next);
+        p->s= next;
         return NAN;
     }
     p->s++; // "("