Fix #74267: segfault with streams and invalid data

If the current character is a line break character, it cannot be a tab
or space character, so we would always fail with an invalid sequ

Fix #74267: segfault with streams and invalid data

If the current character is a line break character, it cannot be a tab
or space character, so we would always fail with an invalid sequence
error. Obviously, these `scan_stat == 4` conditions are meant to be
exclusive.

Furthermore, if `in_pp == NULL || in_left_p == NULL` is true, we hit a
segfault if we are not returning right away. Obviously, the additional
constraints don't make sense, so we remove them.

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