ftrace: allow NULL pointers in mcount_loc

Impact: make ftrace_convert_nops() more permissive

Due to the way different architecture linkers combine the data sections
of the mcount_loc (the section that lists all the locations that
call mcount), there may be zeros added in that section. This is usually
due to strange alignments that the linker performs, that pads in zeros.

This patch makes the conversion code to nops skip any pointer in
the mcount_loc section that is NULL.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>