dfilter: Handle missing error location more gracefully

If we don't have an offset, don't print anything with underline.

Also it can underline filters using macros correctly now.

$ tshark -Y 'ip and ${private_ipv4:ip.sr}' -r /dev/null
tshark: Left side of "==" expression must be a field or function, not "ip.sr".
    ip and ip.sr == 192.168.0.0/16 or ip.sr == 172.16.0.0/12 or ip.sr == 10.0.0.0/8
           ^~~~~

1 files changed, 5 insertions, 3 deletions

diff --git a/dftest.c b/dftest.c
index 6a495e9b82..331a0f9e4b 100644
--- a/dftest.c
+++ b/dftest.c
@@ -167,9 +167,11 @@ main(int argc, char **argv)
     if (!dfilter_compile_real(expanded_text, &df, &err_msg, &err_loc,
                                                 "dftest", TRUE, FALSE)) {
         fprintf(stderr, "dftest: %s\n", err_msg);
-        fprintf(stderr, "\t%s\n", expanded_text);
-        fputc('\t', stderr);
-        putloc(stderr, err_loc.col_start, err_loc.col_len);
+        if (err_loc.col_start >= 0) {
+            fprintf(stderr, "\t%s\n", expanded_text);
+            fputc('\t', stderr);
+            putloc(stderr, err_loc.col_start, err_loc.col_len);
+        }
         g_free(err_msg);
         g_free(expanded_text);
         epan_cleanup();