Fix overflow when checking EOF

[DavidKorczynski]

The fuzzer added in #5 finds an overflow fairly fast, with the following stacktrace:

==13==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x502000014114 at pc 0x55c3024c35fc bp 0x7fff7b16ca30 sp 0x7fff7b16ca28                                                                                                                                             
READ of size 1 at 0x502000014114 thread T0                                                                                                                                                                                                                                         
SCARINESS: 12 (1-byte-read-heap-buffer-overflow)                                                                                                                                                                                                                                   
    #0 0x55c3024c35fb in sql3lexer_next /src/sqlite-createtable-parser/sql3parse_table.c:435:15                                                                                                                                                                                    
    #1 0x55c3024c0656 in sql3parse /src/sqlite-createtable-parser/sql3parse_table.c:1240:25
    #2 0x55c3024c0656 in sql3parse_table /src/sqlite-createtable-parser/sql3parse_table.c:1546:23
    #3 0x55c3024b5316 in LLVMFuzzerTestOneInput /src/fuzzer.c:24:25      

The input to sqlparse_table is: "/**"

This fixes it by extending IS_EOF to also check if the offset has increased beyond the buffer. Can confirm the fuzzer can run now for an extended period of time without findings issues.

[marcobambini]

Thanks @DavidKorczynski!