Fix CID 33235: Argument cannot be negative

src/sbbs3/writemsg.cpp

@@ -1278,7 +1278,7 @@ bool sbbs_t::editfile(char *fname, bool msg)
 	}
 	if((file=nopen(fname,O_RDONLY))!=-1) {
 		length=(long)filelength(file);
-		if(length>(long)maxlines*MAX_LINE_LEN) {
+		if(length < 0 || length>(long)maxlines*MAX_LINE_LEN) {
 			close(file);
 			free(buf); 
 			attr(cfg.color[clr_err]);