Work around GCC -O3 SIGFPE

[sushma-4]

The GEOS build was successful with GCC 13.3.0 on our platform, however two unit tests were failing at runtime with SIGFPE:

• testFEMConsistency
• testFEMConsistency

***** ExternalError
***** Rank 03
***** Message from Signal (detected from Signal Handler):
Floating point error encountered:
Unknown reason.

So I rebuilt the application few ways:

1. Built in debug mode - No erros
2. Release build with -O2 instead of -O3 - no errors
3. Release build using -g for debug info:
   • The FEM consistency tests reproduced the same SIGFPE.
   • Running the failing case under gdb localized the failure to FieldSpecificationEqual::SpecifyFieldValue while applying displacement boundary conditions.

The failure occurred in: cpp8if( entries[ j ] >= 0 && entries[ j ] < minDiagonal )

Using a temporary diagnostic print, I confirmed that the matrix entry involved in the failing case was finite and negative,
Since this value should not mathematically raise a floating-point exception during the comparison, the issue appears to be specific to the GCC 13.3.0 Release -O3 optimization/code-generation path.

This PR introduces a narrow workaround by materializing the matrix entry into a volatile scalar before performing the comparison using volatile