msm: kgsl: Return NULL when done parsing the profiler assignments string

The assignment string parser has a corner condition wherein the user
can craft a malicious string that contains a \0 and force a buffer
overrun.  Avoid this by always returning NULL when the parser encounters
a \0.

CRs-fixed: 547601
Change-Id: Ic0dedbad946e597a5a245df41239b89f114ba9ee
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>