mp_fdjac2: guard non-finite numerical derivatives

The only protection against a non-finite finite-difference derivative
here was assert(isfinite(fjac[ij])), which a release build (-DNDEBUG)
strips out entirely, and the two-sided derivative path had no check at
all. If the user function returns a degenerate residual for some
parameter value, wa[i]/fvec[i] can be non-finite, writing NaN/Inf
straight into fjac.

That NaN then poisons fnorm/ratio in mpfit()'s outer Levenberg-Marquardt
loop. Since any comparison against NaN is false, the iteration counter
(which only advances inside the ratio>=p0001 success branch) never
increments, so the maxiter check that's supposed to bound the loop never
fires. The result is an unbounded CPU spin instead of a clean failure
return -- in our case, confirmed via gdb on a real reproduction: ~465k
spin iterations/sec with zero progress, instead of returning MP_MAXITER.

Fix: sanitize fjac to 0 on a non-finite derivative in all three
non-debug/debug, one-sided/two-sided paths, so the column is treated as
having no measurable gradient -- the same fallback mp_qrfac already uses
for a zero-norm column.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: rocketmark

redist/mpfit/mpfit.c

@@ -1228,14 +1228,16 @@ static int mp_fdjac2(mp_func funct, int m, int n, int *ifree, int npar, FLT *x,
 					/* Non-debug path for speed */
 					for (i = 0; i < m; i++, ij++) {
 						fjac[ij] = (wa[i] - fvec[i]) / h; /* fjac[i+m*j] */
-						assert(isfinite(fjac[ij]));
+						if (!isfinite(fjac[ij]))
+							fjac[ij] = 0;
 					}
 				} else {
 					/* Debug path for correctness */
 					for (i = 0; i < m; i++, ij++) {
 						FLT fjold = fjac[ij];
 						fjac[ij] = (wa[i] - fvec[i]) / h; /* fjac[i+m*j] */
-						assert(isfinite(fjac[ij]));
+						if (!isfinite(fjac[ij]))
+							fjac[ij] = 0;
 						if ((da == 0 && dr == 0 && (fjold != 0 || fjac[ij] != 0)) ||
 							((da != 0 || dr != 0) && (fabs(fjold - fjac[ij]) > da + fabs(fjold) * dr))) {
 							fprintf(stderr, "   %10d %10.4g %10.4g %10.4g %10.4g %10.4g\n", i, fvec[i], fjold, fjac[ij],
@@ -1264,6 +1266,8 @@ static int mp_fdjac2(mp_func funct, int m, int n, int *ifree, int npar, FLT *x,
 					/* Non-debug path for speed */
 					for (i = 0; i < m; i++, ij++) {
 						fjac[ij] = (wa2[ij] - wa[i]) / (2 * h); /* fjac[i+m*j] */
+						if (!isfinite(fjac[ij]))
+							fjac[ij] = 0;
 					}
 				} else {
 					/* Debug path for correctness */