Couchbase: fix Extended value string 'opcode_vals' forced to fall back to linear search

entry 85, value 128 [0x80] < previous entry, value 245 [0xf5]

Change-Id: I14bbe276eef5fdaa70561764ac4e6f2e174ceabe
Reviewed-on: https://code.wireshark.org/review/5570
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>