CLEANUP: standard: Use len2mask4 in str2mask

The len2mask4 function was introduced in commit:
70473a5f8c56d8ec2e837b9b66443dc252b24da9
which is about six years later than the commit that introduced the
str2mask function:
2937c0dd20f2f3c0065b671bbfe3fafcd8862eaf

This is a clean up in preparation for a str2mask6 function which
will use len2mask6.
1 file changed