declare checks on valid nameref names don't account for +=

[Grisha Levit]

The following should probably work:

$ declare -n ref=var ref+=[@]
bash: declare: [@]: invalid variable name for name reference

But the following should not:

$ declare -n ref=re ref+=f
$ declare -p refdeclare -n ref="ref"

The latter causes a segfault for export/readonly.
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