Various UB warnings found by UBSan

[StanFromIreland]

Bug report

Bug description:

These were all found by UBSan in our CI (it just hasn't been complaining about them). They need to be individually checked, some may be false positives.

# Objects/object.c:97:5: runtime error: member access within null pointer of type 'PyThreadState' (aka 'struct _ts')
null:Objects/object.c

# Objects/memoryobject.c:3032:15: runtime error: load of value 2, which is not a valid value for type 'bool'
bool:Objects/memoryobject.c

# Modules/_ctypes/cfield.c:644:1: runtime error: left shift of 1 by 63 places cannot be represented in type 'int64_t' (aka 'long')
shift-base:Modules/_ctypes/cfield.c

# Modules/_ctypes/cfield.c:640:1: runtime error: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'
signed-integer-overflow:Modules/_ctypes/cfield.c

# Modules/_zstd/decompressor.c:598:56: runtime error: applying non-zero offset 18446744073709551615 to null pointer
pointer-overflow:Modules/_zstd/decompressor.c

# Modules/_io/stringio.c:350:24: runtime error: addition of unsigned offset to 0x7fd01ec25850 overflowed to 0x7fd01ec2584c
pointer-overflow:Modules/_io/stringio.c

# Objects/bytesobject.c:1190:25: runtime error: applying zero offset to null pointer
pointer-overflow:Objects/bytesobject.c

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response