Fix gcc warnings when building with optimizations.

[clalancette]

When building the allocator_tutorial_pmr demo with -O2, gcc is throwing an error saying that new and delete are mismatched. This is something of a misnomer, however; the real problem is that the global new override we have in that demo is actually implemented incorrectly.

In particular, the documentation at https://en.cppreference.com/w/cpp/memory/new/operator_new very clearly specifies that operator new either has to return a valid pointer, or throw an exception on error. Our version wasn't throwing the exception, so change it to throw std::bad_alloc if std::malloc fails.

While we are in here, also fix another small possible is where std::malloc could return nullptr on a zero-sized object, thus throwing an exception it shouldn't.

[clalancette]

CI:

• Linux
• Linux-aarch64
• Windows

[clalancette]

CI:

• Linux
• Linux-aarch64
• Windows
• Clang

[clalancette]

CI:

• Linux
• Linux-aarch64
• Windows
• Clang

[ahcorde]

https://github.com/Mergifyio backport jazzy

[mergify[bot]]

backport jazzy

✅ Backports have been created

* [#673 Fix gcc warnings when building with optimizations. (backport #672)](https://github.com/ros2/demos/pull/673) has been created for branch `jazzy`