fbdev: sisfb: Fix strbuf array overflow

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USN-4185-1: Linux kernel vulnerabilities

Stephan van Schaik, Alyssa Milburn, Sebastian Österlund, Pietro Frigo, Kaveh Razavi, Herbert Bos, Cristiano Giuffrida, Giorgi Maisuradze, Moritz Lipp, Michael Schwarz, Daniel Gruss, and Jo Van Bulck discovered that Intel processors using Transactional Synchronization Extensions TSX could expose...