perf script report: Fix SEGFAULT when using DWARF mode

When running perf script report with a Python script and a callgraph in
DWARF mode, intr_regs->regs can be 0 and therefore crashing the regs_map
function.

Added a check for this condition (same check as in builtin-script.c:595).

Signed-off-by: Andreas Gerstmayr <agerstmayr@redhat.com>
Tested-by: Kim Phillips <kim.phillips@amd.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: http://lore.kernel.org/lkml/20200402125417.422232-1-agerstmayr@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>