Fix ascii reallocation in Union_mesh

[mads-bertelsen]

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Please describe what your PR is adding in terms of features or bugfixes:
Fixed small error that exit if reallocation of memory in Union_mesh is successful, rather than exiting if it fails.

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Development OS / boundary conditions

Please describe what OS you developed and tested your additions on, and if any special dependencies are required:
OS X 14.8.7

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PR Checklist for contributing to McStas/McXtrace

For a coherent and useful contribution to McStas/McXtrace, please fill in relevant parts of the checklist:

• My contribution includes patches to an existing component file

  • I have used the mcdoc utility and rendered a reasonable documentation page for the component (please attach as screenshot in comments!)
  • I have ensured that basic use of the component is OK (e.g. an instrument using it compiles?)
  • I have used the mctest utility to test one or more instruments making use of the component (please attach mcviewtest report as screenshot in comments)
  • I have used the mccode-clangformat tool to apply the standard McCode component indentation scheme
  • I have used the mcrun --c-lint "linter" and followed advice to remove most / all warnings that are raised

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[mads-bertelsen]

Geometry_test   :   7.94
Many_meshes     :   5.97
Test_mesh_boxes :   5.43

Running tests / getting status...
Geometry_test    :   Display OK (4s)
Geometry_test    :  11.35    [val: 46.7433 / 47.5 = 98 %]
Geometry_test_2  :  10.92    [val: 46.7433 / 47.5 = 98 %]
Many_meshes      :   Display OK (2s)
Many_meshes      :  14.72    [val: 4.67285e-06 / 4.63384e-06 = 101 %]
Test_mesh_boxes  :   Display OK (2s)
Test_mesh_boxes  :   3.62    [val: 10.7755 / 10.7752 = 100 %]
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Overall test result:
SUCCESS

[willend]

Nice, looks good, merging.