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[PATCH ALTERNATE] builtins: kill: Do not set signal on current pgroup when pid is empty

This was first noticed in the `kill` builtin:

    % zsh; echo "$?"
    % trap 'exit 5' TERM
    % kill ''

This behaviour seems more likely to be the result of bugs in programs
(eg. `kill -9 "$unsetvar") rather than be desirable behaviour to me. It
seems unintentional judging by the code and documentation, since it
comes about as a result of the fact that:

- `isanum` returns true for empty strings (since an empty string
  technically only consists of digits and minuses...);
- `atoi`, when passed a pointer to an invalid number, returns 0;
- kill(0, signal) sends the signal in question to all processes in the
  current process group.

There are two ways to solve this issue:

1. Add special handling to `kill` to avoid this case. See this patch[0]
   for a version that does that.
2. Change how isanum behaves. Since the only two call sites that use it
   both seem like they should handle the case where the input char array
   is empty, that seems like a reasonable overall change to me, but
   either works.

After this patch:

    % trap 'exit 5' TERM
    % kill ''
    kill: illegal pid:

0: https://www.zsh.org/mla/workers/2020/msg00251.html
 Src/jobs.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/Src/jobs.c b/Src/jobs.c
index e7438251e..0485f2c7c 100644
--- a/Src/jobs.c
+++ b/Src/jobs.c
@@ -1854,13 +1854,14 @@ scanjobs(void)
 /* This simple function indicates whether or not s may represent      *
  * a number.  It returns true iff s consists purely of digits and     *
- * minuses.  Note that minus may appear more than once, and the empty *
- * string will produce a `true' response.                             */
+ * minuses.  Note that minus may appear more than once.               */
 static int
 isanum(char *s)
+    if (*s == '\0')
+	return 0;
     while (*s == '-' || idigit(*s))
     return *s == '\0';

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