Fix one-byte over-read in BASE64_DECODE_TABLE (#3547)

The table was declared with 255 entries but indexed by an unsigned
char, so the decoder would read one byte past the array for input
bytes equal to 0xFF. Through jq's normal input paths the over-read
is unreachable because invalid UTF-8 bytes are normalized to U+FFFD
before reaching @base64d, but the table is still wrong at the C
level and reachable by libjq callers that construct strings via
the C API without UTF-8 validation.