gh-151112: Fix crash in compiler_mod() when entering the current compilation unit fails

[StanFromIreland]

_PyCodegen_EnterAnonymousScope can fail to enter the module's compilation unit (e.g. when OOM, like the in repro), so c->u is left NULL. compiler_mod's cleanup then calls _PyCompile_ExitScope, which derefs that NULL unit in compiler_unit_free and segfaults.

The reproducers start/stop for nomemory will probably have to be adjusted, but for me I can reproduce with:

import _testcapi

compile("pass", "<warmup>", "single")
_testcapi.set_nomemory(31, 32)
try:
    compile("pass", "<trigger>", "single")
except MemoryError:
    pass
first = bytes(128)
second = bytes(128)

Skipping news, the other PR has one which I think can cover this too, these are both quite rare.

• Issue: Double free corruption when allocation failures occur in assemble_init #151112

[iritkatriel]

Are we sure c->u was NULL when we entered compiler_mod? Can we assert that at the top of the function?