process: move ecl_clear_bignum_registers to _dealloc_env

This resolves a fixme.

src/c/main.d

@@ -134,6 +134,7 @@ _ecl_dealloc_env(cl_env_ptr env)
   if (!VirtualFree(env, 0, MEM_RELEASE))
     ecl_internal_error("Unable to deallocate environment structure.");
 #else
+  ecl_clear_bignum_registers(env);
   ecl_free_unsafe(env);
 #endif
 }

src/c/process.d

@@ -209,8 +209,6 @@ ecl_disown_cpu()
   if (the_env == NULL)
     return;
   ecl_disable_interrupts_env(the_env);
-  /* FIXME this should be part of dealloc. */
-  ecl_clear_bignum_registers(the_env);
 #ifdef ECL_WINDOWS_THREADS
   CloseHandle(the_env->thread);
 #endif