Fix SIGSEV when running "a"()

rt_call_function_n_kw did check for integers but not for strings
being called. Added a check so running "a"() won't SIGSEV but
throw an exception.
Merge branch 'fix_str_call' of github.com:msiemens/micropython into msiemens-fix_str_call
py: Simplified rt_call_function_n_kw.