i2c: acpi: Fix NULL Pointer dereference

If adapter->dev.parent == NULL there is a NULL pointer dereference in
acpi_i2c_install_space_handler and acpi_i2c_remove_space_handler.

This is present since introduction of this code:
36604751 "i2c: rework kernel config I2C_ACPI" or even
da3c6647

 "I2C/ACPI: Clean up I2C ACPI code and Add CONFIG_I2C_ACPI"

The adapter->dev.parent == NULL case is valid for the i2c_stub,
so loading i2c_stub with ACPI_I2C_OPREGION enabled results in an oops.
This is also valid at least for i2c_tiny_usb and i2c_robotfuzz_osif.

Fix by checking whether it is null before calling ACPI_HANDLE.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>