Fix GH-19088: Make sscanf %c read whitespace

[prateekbhujel]

Fixes GH-19088.

%c already disables the usual leading whitespace skip, but after that it still reused the %s scan path, which stops as soon as it sees whitespace. That made sscanf(' ', '%c%n', ...) assign an empty string and leave the offset at 0.

Keep %c separate from %s in the scan loop so it consumes the requested number of characters, including spaces. The same scanner is used by fscanf(), so the existing %c expectations there are updated as well.

Tests run:

sapi/cli/php run-tests.php -q ext/standard/tests/strings/gh19088.phpt
sapi/cli/php run-tests.php -q ext/standard/tests/strings/gh19088.phpt ext/standard/tests/strings/sscanf_basic4.phpt ext/standard/tests/strings/bug21730.phpt ext/standard/tests/file/fscanf_variation20.phpt ext/standard/tests/file/fscanf_variation21.phpt ext/standard/tests/file/fscanf_variation22.phpt ext/standard/tests/file/fscanf_variation23.phpt ext/standard/tests/file/fscanf_variation24.phpt ext/standard/tests/file/fscanf_variation25.phpt ext/standard/tests/file/fscanf_variation26.phpt
git diff --check