CGNS import fails in multizone simulations

[ChristianBauerEng]

Describe the bug
I'm attempting to run coupled CHT simulations with a single fluid and a single solid zone. Both zones are read as seperate CGNS files.
In Dry-run mode everything works fine. However, when starting an actual run no matter in which order the zones are specified in the configuration list, the 2nd zone is always considered invalid:

------------------- Geometry Preprocessing ( Zone 1 ) -------------------
Reading the CGNS file: MeshSolid.cgns.
WARNING: The CGNS file version (3.31) is old and may cause high memory usage issues, consider updating the file with the cgnsupdate tool.
CGNS file contains 1 database(s).
Database 1, BASE#1: cell dimension of 2, physical dimension of 3.
1 total zone(s).
Zone number 2 invalid

It seems this error is thrown during the CGNS import.

SU2/externals/cgns/cgns_internals.c

Lines 10186 to 10198 in 72b2fa9

	cgns_zone *cgi_get_zone(cgns_file *cg, int B, int Z)
	{
	cgns_base *base;
	base = cgi_get_base(cg, B);
	if (base==0) return CG_OK;
	if (Z>base->nzones || Z<=0) {
	cgi_error("Zone number %d invalid",Z);
	return CG_OK;
	}
	return &(base->zone[Z-1]);
	}

Both zones run fine individually (when adding the missing, coupled BC of course).
When the second zone is converted to SU2 format and read in the native format, the case runs normally.

To Reproduce
Please provide a config file and mesh for a minimal example that recreates the problem.
CoupledSim.zip

Desktop (please complete the following information):

• OS: SUSE Linux Enterprise Server 15 SP1
• C++ compiler and version: intel 19.0.5.281
• MPI implementation and version: intel MPI 2019
• SU2 Version: v7.3.0 (develop)

[pcarruscag]

Literally a one-liner bug, I'll open a PR soon.
Thank you for making it easy to reproduce 👍