Initialise the fetch-into-temporary locals

[heitbaum]

gcc 16 warns -Wmaybe-uninitialized on all six fetch-into-temporary sites in TinyEXIF.cpp:

TinyEXIF.cpp:895:51: warning: 'altitudeRef' may be used uninitialized [-Wmaybe-uninitialized]
TinyEXIF.cpp:765:62: warning: '_FocalLengthIn35mm' may be used uninitialized [-Wmaybe-uninitialized]
TinyEXIF.cpp:727:47: warning: '_ImageHeight' may be used uninitialized [-Wmaybe-uninitialized]
TinyEXIF.cpp:718:46: warning: '_ImageWidth' may be used uninitialized [-Wmaybe-uninitialized]
TinyEXIF.cpp:560:53: warning: '_RelatedImageWidth' may be used uninitialized [-Wmaybe-uninitialized]
TinyEXIF.cpp:569:54: warning: '_RelatedImageHeight' may be used uninitialized [-Wmaybe-uninitialized]

They are false positives. Fetch() writes its out-parameter only on the return true path, and SetFieldIf() returns fetched unchanged, so a temporary is only read when it was written:

bool Fetch(uint16_t& val) const {
    if (!IsShort() || length == 0)
        return false;
    val = parse16(buf + offs + 8, alignIntel);
    return true;
}

bool EXIFInfo::SetFieldIf(FieldID id, bool fetched) {
    if (fetched)
        SetField(id);
    return fetched;
}

gcc just doesn't carry that correlation across the out-of-line SetFieldIf() call. Initialising the temporaries costs nothing and silences the diagnostic.

altitudeRef needs one extra step: it is declared directly under case 5: with no enclosing block, so adding an initialiser there would make the jumps to case 6: and the later labels cross an initialisation (the declaration is legal today only because it has vacuous initialisation). It is wrapped in a block covering just the declaration and its use, leaving the case 5: label and its break; exactly as they are.

Found building the vendored copy of TinyEXIF in xbmc/imagedecoder.heif (synced to 8c22aff) with gcc 16.2.0 for LibreELEC.