pixelpipe: check if `piece' is null to avoid crashes

[agrn]

pixelpipe_get_histogram_backbuf() can be called with piece set to NULL, which can lead to crashes under some conditions (e.g. when the crop filter is active) as it is directly dereferenced without checking its validity. Such a crash is mentioned in issue #357.

This PR adds a null pointer check before dereferencing piece, making the function return early if it is not successful.

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[Jiyone]

This fix works 👍🏻