opj_compress: fix stack buffer overflow in -OutFor option parsing (#1642)

[steps-re]

Fixes #1642.

Bug

parse_cmdline_encoder() copies the user-controlled -OutFor argument into a fixed 50-byte stack buffer with an unbounded sprintf:

char outformat[50];
char *of = opj_optarg;
sprintf(outformat, ".%s", of);

Any -OutFor value longer than 48 chars overflows outformat, corrupting adjacent stack variables. The value comes straight from the command line, so this is a straightforward stack buffer overflow (src/bin/jp2/opj_compress.c:710-712), still present on HEAD.

Fix

Reject over-long values through the existing "Unknown output format" error path (using the file's existing opj_strnlen_s), and use a bounded snprintf as defense-in-depth. Valid values (j2k, j2c, jp2) are unaffected.

Verification

• Built with -fsanitize=address. Before: a 200-char -OutFor → AddressSanitizer: stack-buffer-overflow WRITE of size 200 ... parse_cmdline_encoder opj_compress.c:712. After: same input → clean Unknown output format message, exit 1, ASan clean; clean at the 48-char boundary too.
• Valid -OutFor jp2 / -OutFor j2k still produce correct output files (verified JP2 signature box / j2k output), ASan-clean.