dns: Handle active DNS entries before clearing cache

Issue:
A bug was identified where calling dns_clear_cache() after a DNS query request
but before the query response is received causes the dns_clear_cache() function
to clear the dns_table database. This results in the netconn layer
waiting indefinitely for the dns_call_found() callback, leading to a deadlock.

Resolution:
Added logic to invoke dns_call_found() for any active DNS entries before
clearing the entire DNS cache in the dns_clear_cache() function.
This change ensures that in-use entries are handled properly,
preventing the system from entering a deadlock state.

src/core/dns.c

@@ -408,6 +408,13 @@ dns_setserver(u8_t numdns, const ip_addr_t *dnsserver)
 void
 dns_clear_cache(void)
 {
+  u8_t i=0;
+  for (i = 0; i < DNS_TABLE_SIZE; i++) {
+    if (dns_table[i].state != DNS_STATE_UNUSED) {
+      dns_call_found(i, NULL);
+    }
+  }
+
   memset(dns_table, 0, sizeof(struct dns_table_entry) * DNS_TABLE_SIZE);
 }