BUILD: tools: potential null pointer dereference in dl_collect_libs_cb

This patch fixes a warning that can be reproduced with gcc-8.5 on RHEL8
(gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-28)).

This should fix issue haproxy#3303.

Must be backported everywhere 917e82f ("MINOR: debug: copy debug
symbols from /usr/lib/debug when present") was backported, which is
to branch 3.2 for now.

(cherry picked from commit 33041fe)
Signed-off-by: William Lallemand <wlallemand@haproxy.com>
(cherry picked from commit cd2eaea)
Signed-off-by: William Lallemand <wlallemand@haproxy.com>

Author: wlallemand

src/tools.c

@@ -6002,6 +6002,9 @@ static int dl_collect_libs_cb(struct dl_phdr_info *info, size_t size, void *data
 		/* else it's a VDSO or similar and we're not interested */
 		goto leave;
 
+	if (!fname)
+		goto leave;
+
 	load_file_into_tar(&ctx->storage, &ctx->size, ctx->prefix, fname, NULL, "haproxy-libs-dump");
 
 	/* try to load equivalent debug symbols for absolute paths  */