Avoid unused variable warning in ServerKeyExchange parsing

ssl_server_key_exchange_parse() is compiled even if there's no ciphersuite
enabled which uses it (for example, that's the case in RSA-only builds).
The rationale for that is to avoid cluttering the code with numerous
compile-time guards. A consequence, however, is the top of
ssl_server_key_exchange_parse() contains declarations for variables
which are never put to use, and rightfully leading to compiler warnings.

This commit silences these warnings by putting `((void) VAR);` statements
in the branch which detects if we ever happen to call the function in an
unexpected ciphersuite.

library/ssl_cli.c

@@ -2892,6 +2892,10 @@ static int ssl_server_key_exchange_parse( mbedtls_ssl_context *ssl,
     else
 #endif /* MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED */
     {
+        ((void) ret);
+        ((void) p);
+        ((void) end);
+        ((void) ciphersuite_info);
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "should never happen" ) );
         return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
     }