Segfault on calling neo4j_free_tx with null pointer

[johannessen]

Calling free() with a null pointer should be a no-op in C. But neo4j_free_tx() currently tries to dereference the given pointer without verifying it’s valid, which causes a segmentation fault.

neo4j_transaction_t *tx = 0;
neo4j_free_tx(tx);
// Segmentation fault

If the given pointer is null, neo4j_free_tx() should simply return immediately, just like neo4j_config_free() correctly does:

libneo4j-omni/lib/src/client_config.c

Lines 178 to 183 in b702260

	void neo4j_config_free(neo4j_config_t *config)
	{
	if (config == NULL)
	{
	return;
	}