ftp: make the MDTM date parser stricter (again)

[bagder]

A previous refactor made the parser more lenient and this takes it back to making sure only ascii digits are accepted.

Added test 1684 to verify

Follow-up to 304b518

Pointed out by Codex Security

[bagder]

This is not helpful...

curl/lib/ftp.c:2498:10: error: '*' has higher precedence than '+';
add parentheses to explicitly specify the order  of operations
 [readability-math-missing-parentheses,-warnings-as-errors]
 2498 |   *val = curlx_hexval(p[0]) * 10 + curlx_hexval(p[1]);
      |          ^
      |          (                      )

[Copilot]

Pull request overview

This PR tightens libcurl’s FTP MDTM (213) timestamp parsing to reject non-ASCII digits (addressing a regression toward leniency) and adds a regression test to ensure malformed MDTM dates no longer affect time-condition behavior.

Changes:

• Make the FTP 213 date parser require ASCII digits for each 2-digit field (instead of implicitly accepting other characters).
• Refactor the 2-digit parsing helper to return an error status and write the parsed value via an out-parameter.
• Add test1684 and register it in the test data Makefile to validate malformed MDTM handling.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/data/test1684	Adds an FTP test case with a malformed MDTM timestamp to ensure stricter parsing and expected transfer behavior under -z.
tests/data/Makefile.am	Registers the new test1684 in the test list.
lib/ftp.c	Tightens MDTM “213” date parsing by validating digits with ISDIGIT() before converting fields.

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