Remove a needless memset(). It's okay that we don't append NUL because len is...

Remove a needless memset(). It's okay that we don't append NUL because len is actually one more than the length of argv[0]. However, this is precarious and should probably be replaced with more robust logic.

src/rt/rust_builtin.c

@@ -328,7 +328,6 @@ int rust_get_argv_zero(void* p, size_t* sz)
     return -1;
   }
 
-  memset(p, 0, len);
   memcpy(p, argv[0], len);
   free(argv);
   return 0;