Replacing string += "." by '.'

[Paolo Carlini]

Wolfgang Bangerth wrote:

>I don't know how to test the patch. In case someone would like to consider 
>it, please test it according to the your principles before applying it.
>
Hi Wolfgang.

In order to test it,  just run "make check" inside the libstsdc++-v3 
build directory.

If the testing completes ok (no regressions with the patch applied), you 
should provide a Changelog entry too.

Anyway, the patch looks fine and safe to me.

Ciao, Paolo.