Bound charset detector lookahead - #44
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Charset detection advanced past the NUL terminator when a multibyte sequence ended at a lead byte. This allowed truncated file contents to trigger a heap out-of-bounds read. Check for the terminator before each detector lookahead so incomplete sequences stop without reading beyond the input buffer.
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Charset detection reads one byte beyond the NUL terminator when a multibyte sequence ends at a lead byte. A crafted file can trigger a heap out-of-bounds read during automatic charset detection, potentially crashing Leafpad.
This adds a helper that checks for the NUL terminator before each multibyte lookahead across the Chinese, Japanese, Korean, and escape-sequence detectors.
Validated with ASan and truncated-byte sweeps across the affected locale paths.