Added check of SEXPTYPE and R_NilValue

src/rmatio.c (all_strings_have_equal_length): The function is renamed to
check_string_lengths. Check that elmt != R_NilValue and that
TYPEOF(elmt) equals STRSXP. The return value is now success (0) or
failure (1). The argument equal_length equals 1 if all strings are of
equal length else 0.