fix: heap overflow in fit26 - x[1] written into a 1-element array

DecayFit26::fit writes the fitted fraction to x[0] and the complementary
fraction to x[1], but the Fit26 Python wrapper allocated a 1-element
parameter array (and the SWIG wrapper enforced exactly length 1), so every
fit26 call wrote 8 bytes past the numpy buffer. The overflow was absorbed
silently by allocator slack on macOS/Linux but corrupted the CRT heap on
Windows, crashing the test suite later with 0xc0000374 (detected in
whatever allocation-heavy code ran next).

The parameter array now has both slots, the SWIG wrapper requires at
least length 2, and the complementary fraction is returned to the caller.

Author: tpeulen

ext/python/DecayFit26.i

@@ -17,10 +17,13 @@
     }
 
     static double my_fit(double* x, int n_x, short* fixed, int n_fixed, DecayFitData* p){
-        if (n_x != 1) {
+        // fit26 writes the fitted fraction to x[0] and the complementary
+        // fraction to x[1]; a 1-element array would overflow (heap corruption)
+        if (n_x < 2) {
             PyErr_Format(PyExc_ValueError,
-                         "The length of the parameter vector must of length 1. "
-                         "Arrays of length (%d) given",
+                         "The parameter vector must be at least of length 2 "
+                         "(x[0] fraction in, x[1] complementary fraction out). "
+                         "Array of length (%d) given",
                          n_x);
             return 0.0;
         }

ext/python/Fit2x.py

@@ -365,7 +365,9 @@ def __call__(
                 "length 5 to specify the fixed state for all 6 model "
                 "parameters."
             )
-        x = np.zeros(1, dtype=np.float64)
+        # x[0]: fraction of pattern 1 (in/out); x[1]: 1 - x[0] (output).
+        # fit26 writes both - a 1-element array would overflow.
+        x = np.zeros(2, dtype=np.float64)
         x[0] = initial_values[0]
         if self._verbose:
             print("Fitting")

test/python/decayfit/test_DecayFit26.py

@@ -62,7 +62,7 @@ def test_fit26(self):
         }
         fit26 = tttrlib.Fit26(**settings)
         x1 = 0.5
-        x = np.array([x1])
+        x = np.array([x1, 0.0])  # x[1] receives the complementary fraction
         fixed = np.array([0])
         r = fit26(
             data=data,