grib_lex: use atoll for unsigned 32bit value

[mtasaka]

On 32 bit system, unsigned 32 bit value cannot be read with atol. Use atoll and cast the result to long.

The proposal patch fixes the following test failure on linux 32 bit:

# 193 - eccodes_t_grib_to_netcdf

Forwarded from: https://jira.ecmwf.int/browse/SUP-3560