fix: change calloc parameter order

[Swiftyhu]

Relevant forum post: https://forums.fedoraforum.org/showthread.php?331882-Latest-gcc-compiler

[kzyapkov]

This currently breaks the build of https://aur.archlinux.org/packages/dump1090-fa-git

[mutability]

It's splitting hairs, but if recent gcc is going to complain then okay.

(AFAIK, swapping parameter order to calloc makes no functional difference -- calloc(x,y) returns at least x*y zeroed bytes, with alignment suitable for any type)

[calimeroteknik]

Will this get merged into the master branch as well?

[mutability]

Yes, when we next do a release.