scripts/kallsyms: Fix build failure by setting errno before calling getline()

getline() returns -1 at EOF as well as on error. It also doesn't set
errno to 0 on success, so initialize it to 0 before using errno to check
for an error condition. See the paragraph here [1]:

For some system calls and library functions (e.g., getpriority(2)),
-1 is a valid return on success. In such cases, a successful return
can be distinguished from an error return by setting errno to zero
before the call, and then, if the call returns a status that indicates
that an error may have occurred, checking to see if errno has a
nonzero value.

Bear has a bug [2] that launches processes with errno set and causes the
following build failure:

$ bear -- make LLVM=1
...
LD .tmp_vmlinux.kallsyms1
NM .tmp_vmlinux.kallsyms1.syms
KSYMS .tmp_vmlinux.kallsyms1.S
read_symbol: Invalid argument

[1]: https://linux.die.net/man/3/errno
[2]: https://github.com/rizsotto/Bear/issues/469

Fixes: 1c975da56a6f ("scripts/kallsyms: remove KSYM_NAME_LEN_BUFFER")
Reviewed-by: Miguel Ojeda <[e-mail]>
Signed-off-by: James Clark <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

scripts/kallsyms.c

···
 	ssize_t readlen;
 	struct sym_entry *sym;
 
+	errno = 0;
 	readlen = getline(buf, buf_len, in);
 	if (readlen < 0) {
 		if (errno) {