CVE-2026-26981: OpenEXR has heap-buffer-overflow via signed integer underflow in ImfContextInit.cpp

A heap-buffer-overflow (OOB read) occurs in the istream_nonparallel_read function in ImfContextInit.cpp when parsing a malformed EXR file through a memory-mapped IStream. A signed integer subtraction produces a negative value that is implicitly converted to size_t, resulting in a massive length being passed to memcpy.