BUG/MINOR: ssl: crtlist_dup_filters() must return NULL with fcount == 0

crtlist_dup_filters() must return a NULL ptr if the fcount number is 0.

This bug was introduced by 2954c47 ("MEDIUM: ssl: allow crt-list caching").
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index ea513b2..e5cee53 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -4656,6 +4656,9 @@
 	char **dst;
 	int i;
 
+	if (fcount == 0)
+		return NULL;
+
 	dst = calloc(fcount + 1, sizeof(*dst));
 	if (!dst)
 		return NULL;