Don't crash when a log message has no category (fixes #251)

[munzzyy]

Fixes #251.

Logger::messageOutput() decides whether a line belongs in the in-app log viewer with:

const auto filterWithoutCategory = !strcmp(context.category, "default");

context.category is a raw const char* and isn't always set. Qt's qErrnoWarning() builds a default-constructed QMessageLogContext and leaves category null, and that context reaches the installed handler unchanged — qt_message_print() forwards it as-is. Since this handler is installed process-wide (both application.cpp and cli.cpp call qInstallMessageHandler), it also receives Qt's own internal warnings, so a null category is reachable.

That's the crash in #251. From the qflipper_gdb.txt attached to that issue:

#0  __strcmp_avx2 () at ../sysdeps/x86_64/multiarch/strcmp-avx2.S:283
#1  Logger::messageOutput (type=QtCriticalMsg, context=@...: {version = 2, line = 0,
    file = 0x0, function = 0x0, category = 0x0}, msg=...) at ../backend/logger.cpp:80
#2  qt_message_print (...)
#3  qt_message_output (...)
#4  qErrnoWarning (msg=... "inotify_add_watch(%ls) failed:") at global/qlogging.cpp:1955

category = 0x0 right there in the frame, and the caller is qErrnoWarning from QInotifyFileSystemWatcherEngine::addPaths — KDE's directory watcher hitting the inotify watch limit while the firmware file dialog walks directories. That matches the reporter's "crashes when browsing files" exactly.

The fix treats a null category the same as "default", which is what Qt does internally for this same check (isDefaultCategory() in qlogging.cpp is !category || strcmp(category, "default") == 0). Filtering behaviour doesn't change: uncategorised messages were already kept out of the log viewer, and they're still written to stderr and the log file a few lines above this check.

I couldn't build qFlipper here, so I verified this from the source and the coredump rather than by running it. To confirm on a machine that can: reproduce #251 (browse for a firmware file on a KDE system near its fs.inotify.max_user_watches limit) and check that it no longer crashes and the message still lands in the saved log.