fix: avoid memory leak when possibly malformed packet is received

[m1nl]

getByIndex in Channels.cpp allocates memory and returns dummy channel whenever chIndex validation fails. Comment implies this may happen when malformed packet is received. The fix changes implementation so static dummyChannel is returned in such case.

• I have tested that my proposed changes behave as described.
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[Copilot]

Pull request overview

This PR updates the channel lookup fallback path to avoid dynamically allocating a “dummy” meshtastic_Channel when an invalid channel index is encountered (e.g., due to a malformed packet), returning a static sentinel channel instead.

Changes:

• Introduces a dummyChannel sentinel with index = -1.
• Updates Channels::getByIndex() to return the sentinel on invalid index rather than allocating via malloc().

Comments suppressed due to low confidence (2)

src/mesh/Channels.cpp:315

• getByIndex() used to memset() the fallback channel on every invalid index access. Returning a persistent dummyChannel without reinitializing means any accidental writes via the returned non-const reference (e.g., setChannel() called with an out-of-range cc.index) can leave stale has_settings/settings data that later malformed packets will observe. Please re-zero the dummy on each invalid access (and then set index = -1), or keep a function-local static dummy that is reset before returning, to preserve the prior semantics while avoiding malloc().

        LOG_ERROR("Invalid channel index %d > %d, malformed packet received?", chIndex, channelFile.channels_count);
        return dummyChannel;
    }

src/mesh/Channels.cpp:26

• dummyChannel is only used within this translation unit, but as written it has external linkage. Consider giving it internal linkage (e.g., static or anonymous namespace), or moving it into getByIndex() as a function-local static, to avoid exporting an unnecessary global symbol.

meshtastic_Channel dummyChannel = {.index = -1};