Revert "exec: return NULL immediately if the task id is invalid"

This reverts commit dff1bf6f0f.

(cherry picked from commit 08ebc99dea)

ChangeLog.adoc

@@ -20,6 +20,7 @@ https://weechat.org/files/releasenotes/ReleaseNotes-devel.html[release notes]
 
 Bug fixes::
 
+  * exec: fix search of command by identifier
   * spell: fix refresh of bar item "spell_suggest" when the input becomes empty (issue #1586)
   * spell: fix crash with IRC color codes in command line (issue #1589)
 

src/plugins/exec/exec.c

@@ -89,7 +89,7 @@ exec_search_by_id (const char *id)
     error = NULL;
     number = strtol (id, &error, 10);
     if (!error || error[0])
-        return NULL;
+        number = -1;
 
     for (ptr_exec_cmd = exec_cmds; ptr_exec_cmd;
          ptr_exec_cmd = ptr_exec_cmd->next_cmd)