Various minor cleanups in net_crypto.

[iphydf]

• Consistently use num_packets_array to get the packet count in the
  packet buffer.
• Use const in more places.
• Typo fix: begginning.
• Rewrite length < 1 for unsigned int to length == 0.
• Limit scope of some loop variables by using for-init-decl.
• Use early return in error paths to reduce indentation and for clarity.
• Use net_unpack_* instead of manual ntohs.
• Fix an uninitialised stack variable copy.
• Fix a potential null pointer dereference.
• Consistently use get_crypto_connection. It was inlined in some
  places. I de-inlined it now.

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This change is 

[robinlinden]

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Reviewed 8 of 8 files at r2.
Review status: 0 of 1 LGTMs obtained (waiting on @sudden6)

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Comments from Reviewable

[sudden6]

Reviewed 1 of 18 files at r1, 1 of 8 files at r2, 1 of 1 files at r3.
Review status: 0 of 1 LGTMs obtained (waiting on @sudden6)

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toxcore/net_crypto.c, line 1106 at r3 (raw file):

    /* If last packet send failed, try to send packet again.
       If sending it fails we won't be able to send the new packet. */
    if (conn->maximum_speed_reached) {

Invert condition and do early return? or do you want to keep the success case at the end of the function?

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toxcore/net_crypto.c, line 1111 at r3 (raw file):

        const int ret = get_data_pointer(c->log, &conn->send_array, &dt, packet_num);

        uint8_t send_failed = 0;

This variable can be removed by moving the return into the if

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Comments from Reviewable

[iphydf]

Review status: 0 of 1 LGTMs obtained (waiting on @sudden6)

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toxcore/net_crypto.c, line 1106 at r3 (raw file):

Previously, sudden6 wrote…

Invert condition and do early return? or do you want to keep the success case at the end of the function?

Yes, I'd rather keep success at the end.

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toxcore/net_crypto.c, line 1111 at r3 (raw file):

Previously, sudden6 wrote…

This variable can be removed by moving the return into the if

Done.

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Comments from Reviewable

[sudden6]

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Reviewed 1 of 1 files at r4.
Review status: complete! 1 of 1 LGTMs obtained

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toxcore/net_crypto.c, line 1106 at r3 (raw file):

Previously, iphydf wrote…

Yes, I'd rather keep success at the end.

Ok, can live with that

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Comments from Reviewable