Travis: Enable UBSan checks in CI

[Algunenano]

Adding a check with both gcc and clang undefined behaviour sanitizer in Travis.

For now, I'll leave out topology since debugging it requires knowing at least how it's supposed to work.

[algunenano]

In 17034:

Travis: Run CI under GCC's undefined behavior sanitizer

References #4244
Closes ​https://github.com/postgis/postgis/pull/335

[Algunenano]

PR adding clang UBsan checks: ​https://github.com/postgis/postgis/pull/337

Thanks to Kompzpa for the help adding Clang to the docker image.

Pending:

• Have a Docker image that compiles Postgresql with clang so clang's ubsan is also run in the extension code (the part using PGXS). This would also be useful in case we want to debug JIT issues, as currently gcc is being used.

• Fix topology undefined behaviour alerts: I might revisit it at some point in the future, but I find other warnings (memory sanitizer) more interesting right now.

[algunenano]

In 17054:

Travis: Run CI under clang's ubsan

References #4244
Closes ​https://github.com/postgis/postgis/pull/337

[Algunenano]

The issue in topology is an unaligned memory access in BOX2D_out, so it was really in the Postgis extension, not topology itself.

Addressed in ​https://github.com/postgis/postgis/pull/342

[algunenano]

In 17056:

Avoid unaligned memory access in BOX2D_out

References #4244

[algunenano]

In 17057:

Avoid unaligned memory access in BOX2D_out

References #4244

[algunenano]

In 17058:

Avoid unaligned memory access in BOX2D_out

References #4244

[algunenano]

In 17059:

Avoid unaligned memory access in BOX2D_out

Also enables UBSan in Travis runs for topology

Closes #4244
Closes ​https://github.com/postgis/postgis/pull/342