job: Fix memory leak in job_start().

If a new job cannot be started because no slots are free, we return early
without freeing the argv argument.

src/nvim/os/job.c

@@ -139,6 +139,7 @@ Job *job_start(char **argv,
 
   if (i == MAX_RUNNING_JOBS) {
     // No free slots
+    shell_free_argv(argv);
     *status = 0;
     return NULL;
   }