bpo-41060: Avoid SEGFAULT when calling GET_INVALID_TARGET in the grammar (GH-21020)

bpo-41060: Avoid SEGFAULT when calling GET_INVALID_TARGET in the grammar (GH-21020)

`GET_INVALID_TARGET` might unexpectedly return `NULL`, which if not
caught will cause a SEGFAULT. Therefore, this commit introduces a new
inline function `RAISE_SYNTAX_ERROR_INVALID_TARGET` that always
checks for `GET_INVALID_TARGET` returning NULL and can be used in
the grammar, replacing the long C ternary operation used till now.