fish: Initialize pcmd structure.

On Debian we get this warning which I'm pretty sure is bogus:

fish.c:690: error: 'pcmd.cmd' may be used uninitialized in this
function [-Wuninitialized]

fish/fish.c

@@ -678,7 +678,10 @@ parse_command_line (char *buf, int *exit_on_error_rtn)
   int r;
   const size_t argv_len = sizeof pcmd.argv / sizeof pcmd.argv[0];
 
-  pcmd.pipe = NULL;
+  /* Note that pcmd.pipe must be set to NULL for correct usage.  Other
+   * fields do not need to be, but this silences a gcc warning.
+   */
+  memset (&pcmd, 0, sizeof pcmd);
 
  again:
   /* Skip any initial whitespace before the command. */