[libc] Fix GPU argument vector writing nullptr to string

Summary:
The intention behind this code was to null terminate the `envp` string,
but it accidentally went into the string data.

Author: jhuber6

libc/utils/gpu/loader/Loader.h

@@ -85,7 +85,7 @@ void *copy_argument_vector(int argc, const char **argv, Allocator alloc) {
   }
 
   // Ensure the vector is null terminated.
-  reinterpret_cast<void **>(dev_argv)[argv_size] = nullptr;
+  reinterpret_cast<void **>(dev_argv)[argc + 1] = nullptr;
   return dev_argv;
 }