Potential NMBS_ERROR_INVALID_ARGUMENT when calling nmbs_create()

[debevv]

Following #55 I realized that there was no way to check if the new crc_calc() function pointer was defined by the user or not, since there is no guarantee that the user initializes the nmbs_platform_conf struct to zero.

For this reason, with #64 (which marks version 1.18.0), I introduced 2 new _create() APIs that the user must call to create the nmbs_platform_conf and nmbs_callbacks struct, nmbs_platform_conf_create() and nmbs_callbacks_create().

To force the usage of these functions inside old codebases, I introduced a runtime check inside nmbs_create() and nmbs_server_create() returning NMBS_ERROR_INVALID_ARGUMENT when one of these structs is not initialized properly.