[examples] fix udpclient argument parsing

[xuh0923]

Why to submit this PR

examples/network/udpclient.c has an argument parsing issue in the --cnt handling path.
The current code checks argc == 7 but then reads argv[7], and it also checks the --cnt option at the wrong index.
For invalid command formats, this can lead to out-of-bounds access during argument parsing.

What is your solution

• Restrict the normal -h <host> -p <port> path to valid argument counts only
• Fix the --cnt option index from argv[6] to argv[5]
• Fix the count value index from argv[7] to argv[6]
• Reject malformed 7-argument input by falling back to usage output

Provide the config and bsp

• BSP: N/A
• .config: N/A
• action: N/A

Verification

• Reviewed the updated parsing flow for:
  • udpclient --help
  • udpclient --stop
  • udpclient -h <host> -p <port>
  • udpclient -h <host> -p <port> --cnt <count>
• Verified the new logic no longer reads beyond the valid argv range for the --cnt path
• Verified the diff only updates examples/network/udpclient.c

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