issue #8139 Segfault after a8bf5d3

As indicated in the issue the proposed fix fixes OP's problem (according to his initial tests) as well as the problem in the CGAL documentation.

Author: albert-github

src/doxygen.cpp

@@ -2446,7 +2446,7 @@ static bool isVarWithConstructor(const Entry *root)
     result=FALSE;
     goto done;
   }
-  else if (fd->name().right(2)==".c" || fd->name().right(2)==".h")
+  else if ((fd != nullptr) && (fd->name().right(2)==".c" || fd->name().right(2)==".h"))
   { // inside a .c file
     result=FALSE;
     goto done;