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: 1c975da5 ("scripts/kallsyms: remove KSYM_NAME_LEN_BUFFER")
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: James Clark <james.clark@arm.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kallsyms.c

@@ -129,6 +129,7 @@ static struct sym_entry *read_symbol(FILE *in, char **buf, size_t *buf_len)
 	ssize_t readlen;
 	struct sym_entry *sym;
 
+	errno = 0;
 	readlen = getline(buf, buf_len, in);
 	if (readlen < 0) {
 		if (errno) {