Fix ax25SendUFrame stack overflow on U-frame length write

[SimonVutov]

Purpose

Closes UWOrbital/obc-firmware#437

Fixes the Mac-specific uFrameSendRecv* test failures (UWOrbital/obc-firmware#437).

Changes

ax25SendUFrame passed a uint8_t struct member as a uint16_t* to ax25Stuff, causing a 1-byte stack buffer overflow. This was tolerated on Linux but caused SIGABRT on macOS, aborting the obc-interface-tests binary.

The fix writes the stuffed length to a temporary uint16_t before assigning it to the uint8_t length field.

Verified against obc-firmware's test suite. Before the fix, TestAx25SendRecv.uFrameSendRecvConn aborted the test binary; afterwards, all three uFrameSendRecv* tests pass.

Testing

Before the fix (the test aborts on uFrameSendRecvConn):

[----------] 6 tests from TestAx25SendRecv
[ RUN      ] TestAx25SendRecv.iFrameLittleStuff
[       OK ] TestAx25SendRecv.iFrameLittleStuff (0 ms)
[ RUN      ] TestAx25SendRecv.iFrameMaxStuff
[       OK ] TestAx25SendRecv.iFrameMaxStuff (0 ms)
[ RUN      ] TestAx25SendRecv.iFrameSomeStuff
[       OK ] TestAx25SendRecv.iFrameSomeStuff (0 ms)
[ RUN      ] TestAx25SendRecv.uFrameSendRecvConn
1/2 Test #1: obc-interface-tests ..............Subprocess aborted***Exception:   0.43 sec
test 2
    Start 2: obc-firmware-tests

After the fix (all three U-frame tests pass):

[----------] 6 tests from TestAx25SendRecv
[ RUN      ] TestAx25SendRecv.iFrameLittleStuff
[       OK ] TestAx25SendRecv.iFrameLittleStuff (0 ms)
[ RUN      ] TestAx25SendRecv.iFrameMaxStuff
[       OK ] TestAx25SendRecv.iFrameMaxStuff (0 ms)
[ RUN      ] TestAx25SendRecv.iFrameSomeStuff
[       OK ] TestAx25SendRecv.iFrameSomeStuff (0 ms)
[ RUN      ] TestAx25SendRecv.uFrameSendRecvConn
[       OK ] TestAx25SendRecv.uFrameSendRecvConn (0 ms)
[ RUN      ] TestAx25SendRecv.uFrameSendRecvDisc
[       OK ] TestAx25SendRecv.uFrameSendRecvDisc (0 ms)
[ RUN      ] TestAx25SendRecv.uFrameSendRecvAck
[       OK ] TestAx25SendRecv.uFrameSendRecvAck (0 ms)
[----------] 6 tests from TestAx25SendRecv (0 ms total)