Detect and replace NaNs in cXyz default ctor

This fixes intermittent segfaults due to invalid atan table lookups.

For some reason zero-initializing all cXyz objects causes issues, as
does initializing them with INFINITY or -INFINITY. However, at least
one of the possible crashes (in d_a_e_yg's search_ground_1) is
guaranteed to happen whenever a specific uninitialized cXyz contains
a NaN for x or z.

A possible explanation for these crashes not occurring on hardware might
be that the problematic objects happen to be placed at memory locations
that happen to never contain a NaN upon allocation, so the buggy code
was never caught. Further investigation would be needed to determine if
this is what's actually happening, though.

include/SSystem/SComponent/c_xyz.h

@@ -31,7 +31,17 @@ struct cXyz : Vec {
         z = vec.z;
     }
 #else
-    cXyz() = default;
+    cXyz() {
+        if (std::isnan(x)) {
+            x = 0.0f;
+        }
+        if (std::isnan(y)) {
+            y = 0.0f;
+        }
+        if (std::isnan(z)) {
+            z = 0.0f;
+        }
+    }
     ~cXyz() = default;
     cXyz(const cXyz& vec) = default;
 #endif