Add a bound check for sas7bcat_parse_value_labels when finding the offset of labels.

[kiwiwarmnfuzzy]

lbp1 will try to chase and read as many value labels as label_count_capacity, which may result in accessing memory locations past the allocated region, especially for malformed data. This also results in Address sanitizer error on some inputs. Add a bounds check to avoid it.

Closes #299
Maybe related to #285

[kiwiwarmnfuzzy]

@evanmiller Hi there! Please take a look when you have time - this addresses one of the vulnerabilities identified by our fuzzing tool; realistically, this can happen with incorrect or malformed data.

[evanmiller]

Hi, I'm just confused because I thought line 63 was performing this check

        if (&lbp1[3] - value_start > value_labels_len || sas_read2(&lbp1[2], ctx->bswap) < 0) {

But maybe I'm missing something?

[evanmiller]

Ah, looks like a regression introduced in #293. I will prepare a fix.