gethostbyname won't accept empty name

Considers both versions - synchronous and asynchronous. Earlier the
underlying stack was trusted to do this check.

Author: Veijo Pesonen

features/netsocket/NetworkStack.cpp

@@ -28,6 +28,10 @@ const char *NetworkStack::get_ip_address()
 }
 nsapi_error_t NetworkStack::gethostbyname(const char *name, SocketAddress *address, nsapi_version_t version)
 {
+    if (name[0] == '\0') {
+        return NSAPI_ERROR_PARAMETER;
+    }
+
     // check for simple ip addresses
     if (address->set_ip_address(name)) {
         if (version != NSAPI_UNSPEC && address->get_ip_version() != version) {
@@ -53,6 +57,10 @@ nsapi_value_or_error_t NetworkStack::gethostbyname_async(const char *name, hostb
 {
     SocketAddress address;
 
+    if (name[0] == '\0') {
+        return NSAPI_ERROR_PARAMETER;
+    }
+
     // check for simple ip addresses
     if (address.set_ip_address(name)) {
         if (version != NSAPI_UNSPEC && address.get_ip_version() != version) {