msm: ADSPRPC: Validate rpra to avoid Null pointer dereference

In get_args function their is no NULL pointer check for rpra
variable, that might lead to null pointer dereference. Add
condition to verify.

Change-Id: I0789e8ea875221de5809598419bc7f842aa3e22e
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>