benchtests: Fix validate_benchout.py exceptions

This patch fixed validate_benchout.py two exceptions,
1) AttributeError
   if benchout_strings.schema.json is specified, and
2) json.decoder.JSONDecodeError
   if benchout file is not JSON.

$ ~/glibc/benchtests/scripts/validate_benchout.py bench-memset.out \
~/glibc/benchtests/scripts/benchout_strings.schema.json
Traceback (most recent call last):
  File "/home/naohirot/glibc/benchtests/scripts/validate_benchout.py", line 86, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/home/naohirot/glibc/benchtests/scripts/validate_benchout.py", line 69, in main
    bench.parse_bench(args[0], args[1])
  File "/home/naohirot/glibc/benchtests/scripts/import_bench.py", line 139, in parse_bench
    do_for_all_timings(bench, lambda b, f, v:
  File "/home/naohirot/glibc/benchtests/scripts/import_bench.py", line 107, in do_for_all_timings
    if 'timings' not in bench['functions'][func][k].keys():
AttributeError: 'str' object has no attribute 'keys'

$ ~/glibc/benchtests/scripts/validate_benchout.py bench-math-inlines.out \
~/glibc/benchtests/scripts/benchout_strings.schema.json
Traceback (most recent call last):
  File "/home/naohirot/glibc/benchtests/scripts/validate_benchout.py", line 86, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/home/naohirot/glibc/benchtests/scripts/validate_benchout.py", line 69, in main
    bench.parse_bench(args[0], args[1])
  File "/home/naohirot/glibc/benchtests/scripts/import_bench.py", line 137, in parse_bench
    bench = json.load(benchfile)
  File "/usr/lib/python3.6/json/__init__.py", line 299, in load
    parse_constant=parse_constant, object_pairs_hook=object_pairs_hook, **kw)
  File "/usr/lib/python3.6/json/__init__.py", line 354, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3.6/json/decoder.py", line 342, in decode
    raise JSONDecodeError("Extra data", s, end)
json.decoder.JSONDecodeError: Extra data: line 1 column 17 (char 16)

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>