Perf: arm64: avoid use of uninitialized variable

Variable "buf" is not initialized. It may contain the data from
stack when "count == 0". Kernel information leak is possible due
to different code path execution.

Change-Id: I041b2e7262f7f66a9b039c0f0591bbb1fc9e4890
Signed-off-by: Patrick Fay <pfay@codeaurora.org>