When index() or rindex() was invoked with a string haystack and a non-
string needle argument, a segmentation fault occurred due to an internal
strlen() invocation on a NULL pointer.

-- Testcase --
print(index("abc", []), "\n")
-- End --

-- Args --
-R
-- End --

-- Expect stdout --
-1
-- End --


-- Testcase --
print(rindex("abc", []), "\n")
-- End --

-- Args --
-R
-- End --

-- Expect stdout --
-1
-- End --