firmware: tegra: bpmp: Return directly after a failed kzalloc() in get_filename()

The kfree() function was called in one case by
the get_filename() function during error handling
even if the passed variable contained a null pointer.
This issue was detected by using the Coccinelle software.

Thus return directly after a call of the function “kzalloc” failed
at the beginning.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Thierry Reding <treding@nvidia.com>