POSIX requires getdelim()/getline() to allocate a new buffer whenever
*lineptr is NULL, regardless of the value of *n. The previous code read
the buffer size from *n unconditionally and only fell back to the initial
size when *n was zero, so a caller that passes *lineptr == NULL together
with an uninitialized (non-zero) *n caused lib_malloc() to be invoked with
that garbage size and typically fail with ENOMEM.
Treat a NULL *lineptr the same as a zero *n: (re)allocate from the known
BUFSIZE_INIT and ignore the untrusted *n. This matches the glibc
behaviour that portable code relies on (for example toybox grep, which
calls getdelim() with an uninitialized size variable).
Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
