Iterate by index in auto_load_safe_path_vec_update

Iterate by index in auto_load_safe_path_vec_update

As reported by Jan, we get this error when building with -D_GLIBCXX_DEBUG:

/usr/include/c++/7/debug/safe_iterator.h:297:
Error: attempt to increment a singular iterator.
Objects involved in the operation:
    iterator "this" @ 0x0x7fffffffd140 {
      type = __gnu_debug::_Safe_iterator<__gnu_cxx::__normal_iterator<std::unique_ptr<char, gdb::xfree_deleter<char> >*, std::__cxx1998::vector<std::unique_ptr<char, gdb::xfree_deleter<char> >, std::allocator<std::unique_ptr<char, gdb::xfree_deleter<char> > > > >, std::__debug::vector<std::unique_ptr<char, gdb::xfree_deleter<char> >, std::allocator<std::unique_ptr<char, gdb::xfree_deleter<char> > > > > (mutable iterator);
      state = singular;
      references sequence with type 'std::__debug::vector<std::unique_ptr<char, gdb::xfree_deleter<char> >, std::allocator<std::unique_ptr<char, gdb::xfree_deleter<char> > > >' @ 0x0x265db40
    }

The bug was introduced by commit

commit e80aaf6183c6692ecc167bf26cbdc53f8f1a55f0
Author: Simon Marchi <simon.marchi@polymtl.ca>
Date:   Fri Mar 2 23:22:06 2018 -0500
Make delim_string_to_char_ptr_vec return an std::vector

The problem is that we iterate using a range-based for on a vector to
which we push in the loop.  Pushing to the vector invalidates the
iterator used in the loop.  Instead, change the code to iterate by index
as was done in the previous code.

gdb/ChangeLog:

	* auto-load.c (auto_load_safe_path_vec_update): Iterate by
	index.