[image output] Initialize pixels of partial tile conversion buffer.

When writing a partial tile, the unused pixels still go through float conversion.

This means, that floating point operations are done on uninitialized data.

This can easily lead to NaN and to floating point exceptions, if those were to be enabled.

This change will set the shared buffer used for all partial tiles to all zero pixels.

Tested by running valgrind before and after the change

FIXES: #4461

Description

[x] I have read the contribution guidelines.
[x] I have updated the documentation, if applicable. (Check if there is no need to update the documentation, for example if this is a bug fix that doesn't change the API.)
[x] I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
[x] If I added or modified a C++ API call, I have also amended the corresponding Python bindings (and if altering ImageBufAlgo functions, also exposed the new functionality as oiiotool options).
[x] My code follows the prevailing code style of this project. If I haven't already run clang-format before submitting, I definitely will look at the CI test that runs clang-format and fix anything that it highlights as being nonconforming.