Fix: LibreSSL PSK error (closes #1569)

Fixes "Invalid PSK settings" error using LibreSSL when no PSK is used

src/mongoose.c

@@ -5042,8 +5042,7 @@ static enum mg_ssl_if_result mg_ssl_if_ossl_set_psk(struct mg_ssl_if_ctx *ctx,
                                                     const char *identity,
                                                     const char *key_str) {
   (void) ctx;
-  (void) identity;
+  if (identity == NULL && key_str == NULL) return MG_SSL_OK;
-  (void) key_str;
   /* Krypton / LibreSSL does not support PSK. */
   return MG_SSL_ERROR;
 }

src/mongoose_libressl.patch

@@ -0,0 +1,14 @@
+diff --git a/src/mongoose.c b/src/mongoose.c
+index d3d2ef99..c9c40863 100644
+--- a/src/mongoose.c
++++ b/src/mongoose.c
+@@ -5042,8 +5042,7 @@ static enum mg_ssl_if_result mg_ssl_if_ossl_set_psk(struct mg_ssl_if_ctx *ctx,
+                                                     const char *identity,
+                                                     const char *key_str) {
+   (void) ctx;
+-  (void) identity;
+-  (void) key_str;
++  if (identity == NULL && key_str == NULL) return MG_SSL_OK;
+   /* Krypton / LibreSSL does not support PSK. */
+   return MG_SSL_ERROR;
+ }