Fix incorrect return-value check for a scanf like function (CWE-253)

[gsnw]

This pull request fix the codeql security CWE-253 alert Incorrect return-value check for a 'scanf'-like function

[coveralls]

coverage: 85.117%. remained the same
when pulling 5d0fdaa on gsnw:fix-scanf-cwe253
into 2f2ff0b on schweikert:develop.

[auerswal]

I have looked at the changes, and reviewed the sscanf man page. This change seems correct, because only the return value of 1 represents matching the single expected value.

A value of 0 would mean that nothing matched the format string. The current code checks against this. A number greater than 1 seems unlikely with a format string that comprises a single conversion specification. In case of an error, EOF could also be returned, but the current code would not catch this, I think.

[gsnw]

Fix issue #324

[auerswal]

sscanf() returns EOF when used with an empty string as input. This can be given as option argument, e.g., using fping -t '' …. Without the pull request, fping does not catch this invalid option argument.

[coveralls]

coverage: 85.54% (+0.07%) from 85.469%
when pulling edffcc3 on gsnw:fix-scanf-cwe253
into 118cdc5 on schweikert:develop.

[auerswal]

I have looked at the changes again, IMHO they are "obviously correct", so I'll just go ahead and merge this.
@schweikert: Please let me know if this is OK.

[schweikert]

Looks good to me as well. Thanks!