oprofile: protect from not being in an IRQ context

oprofile: protect from not being in an IRQ context

http://lkml.org/lkml/2010/4/27/285

Protect against dereferencing regs when it's NULL, and
force a magic number into pc to prevent too deep processing.
This approach permits the dropped samples to be tallied as
invalid Instruction Pointer events.

e.g. output from about 15mins at 10kHz sample rate:
Nr. samples received: 2565380
Nr. samples lost invalid pc: 4

Signed-off-by: Phil Carmody <ext-phil.2.carmody@nokia.com>
Signed-off-by: Robert Richter <robert.richter@amd.com>