debug consistency: fix unused-but-set-variable warning

[hannesbrandt]

@donnaaboise reported the following unused-but-set-variable warning.

/Users/calhoun/projects/ForestClaw/code/forestclaw/src/fclaw2d_convenience.c:1813:9: warning: variable 'prev_rank' set but not used [-Wunused-but-set-variable]
 1813 |     int prev_rank;
      |         ^~~~~~~~~
[183/1751] Building C object src/CMakeFiles/forestclaw_c.dir/fclaw2d_file.c.o

This PR replaces the call SC_ASSERT (prev_rank < sb->rank); by FCLAW_ASSERT (prev_rank < sb->rank); so that prev_rank is set and used consistently in an FCLAW_ENABLE_DEBUG environment.
I was not able to reproduce the compiler warning. Does this solve the warning, @donnaaboise?

There are a few more occurrences of P4EST_ASSERT and P4EST_ENABLE_DEBUG in fclaw2d_convenience.c, forestclaw2d.c,
fclaw2d_p4est.c and patches/clawpatch/fclaw_clawpatch_output_vtk.c. I could go ahead and replace them with their FCLAW counterpart in another commit, if this is okay with you @donnaaboise @cburstedde. Such a change would prevent issues caused by sc/p4est and forestclaw having different debug configurations.

[donnaaboise]

Yes - please go ahead and make the changes FCLAW_XXX asserts. I assume @cburstedde is okay with this?

[cburstedde]

Yes - please go ahead and make the changes FCLAW_XXX asserts. I assume @cburstedde is okay with this?

Sounds good, please @hannesbrandt go ahead with the remaining changes.