Fix the usage of ssl context after its nullified

Previously, it was possible to access a null pointer
even though the given configuration should work.
Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>

library/ssl_tls.c

@@ -1435,7 +1435,8 @@ int ssl_populate_transform( mbedtls_ssl_transform *transform,
 
 #if !defined(MBEDTLS_SSL_HW_RECORD_ACCEL) && \
     !defined(MBEDTLS_SSL_EXPORT_KEYS) && \
-    !defined(MBEDTLS_DEBUG_C)
+    !defined(MBEDTLS_DEBUG_C) && \
+    !defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
     ssl = NULL; /* make sure we don't use it except for those cases */
     (void) ssl;
 #endif