tinyexpr: Check for nan in ncr

Turns out this takes ages.

Fixes #8170

src/tinyexpr.cpp

@@ -156,6 +156,8 @@ static double fac(double a) { /* simplest version of fac */
 }
 
 static double ncr(double n, double r) {
+    // Doing this for NAN takes ages - just return the result right away.
+    if (std::isnan(n)) return INFINITY;
     if (n < 0.0 || r < 0.0 || n < r) return NAN;
     if (n > UINT_MAX || r > UINT_MAX) return INFINITY;
     unsigned long int un = static_cast<unsigned int>(n), ur = static_cast<unsigned int>(r), i;

tests/checks/math.fish

@@ -219,3 +219,9 @@ math pow 2, cos -pi
 # i.e. 4
 math pow 2 x cos'(-pi)', 2
 # CHECK: 4
+
+# This used to take ages, see #8170.
+# If this test hangs, that's reintroduced!
+math 'ncr(0/0, 1)'
+# CHECKERR: math: Error: Result is infinite
+# CHECKERR: 'ncr(0/0, 1)'