firmware: qcom: tz_log: add error checking for ion_map_kernel

ion_map_kernel can return an ERR_PTR but we're currently assuming that
it always returns a valid pointer. Fix this by checking for IS_ERR
before dereferencing the pointer.

Change-Id: I2f606c042ad8c8ff6bc8db6501eed0a0924290fb
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>