There is no attached source code.
Can you attach the source code where you are having the seg fault at runtime?

I bet you have a buffer overflow in your code though.

Created attachment 27166 [details]
C-Source code

C-source code

You are using an uninitialized pointer.

And GCC tells you so:

manuel@gcc12:~$ ~/trunk/186353M/build/gcc/cc1 -Wall tmp.c 
tmp.c:3:6: warning: return type of ‘main’ is not ‘int’ [-Wmain]
 void main()
      ^
tmp.c: In function ‘main’:
tmp.c:8:8: warning: ‘prod_vg’ is used uninitialized in this function [-Wuninitialized]
   multg(prod_vg);
        ^

Now that -Wuninitialized is separated from -Wmaybe-uninitialized, maybe it is time to enable it by default.