Guard arithmetic on file/shell-output-derived values against non-numeric strings

PHP 8 throws a fatal TypeError for arithmetic (+ - * /) on a
non-numeric string ("Unsupported operand types"); PHP 7.4 only warned
and treated it as 0. Several files perform arithmetic directly on
values parsed from intermediate state files (nnode.txt, nrows.txt,
structure_input*.txt) or from `dot -Tplain` output, with no
is_numeric() check — safe as long as those files are well-formed, but
a crash risk if a file is missing/empty/truncated or a `dot` output
line doesn't parse as expected.

Affected: runtime_check.php ($node, $nrows), remove_variables.php and
create_tiers_gom_part1.php ($maxplist=$node-1), mat_structure.php
(matrix cell multiplication), and the four network_layout_{evd,inv}[,_2].php
files ($r_index=$node+N and the dot-output-derived $cell arithmetic in
the _2 variants).

Fix pattern: coalesce to 0 via `is_numeric($x) ? $x : 0` before the
arithmetic, matching PHP 7.4's original fallback-to-0 behavior instead
of crashing.

Verified empirically against PHP 8.3.28 with runtime_check.php and
mat_structure.php: previously, a missing/empty source file caused
  TypeError: Unsupported operand types: string - int (or string * int)
after the fix, the same missing-file scenario produces only the
pre-existing "undefined array key" warnings (same as PHP 7.4) and the
function completes and returns a value instead of crashing.

Reviewed by: Frederick M Muriithi <fredmanglis@gmail.com>

0 files changed, 0 insertions, 0 deletions