FIX: Add safety check for zero reference area (Fixes #2435)

[ayush4874]

Fixes #2435.

If REF_AREA is set to 0.0 (auto-calculation) but the geometry projection fails (e.g. flat plate aligned with projection plane), the area remains 0.0. This causes immediate divergence/NaNs later in the solver.

Added a check in SetPositive_ZArea to catch this case and error out cleanly via SU2_MPI::Error instead of crashing.

Verified using the NACA0012 testcase:

• Validated that a standard run with REF_AREA=0.0 still works correctly (calculates ~1.0), ensuring no regression for valid cases.

[pcarruscag]

Reference area is not necessary for all physics in SU2

[ayush4874]

Should I change this to a SU2_MPI::Warning, or should I move the check to where the coefficients are actually calculated?