Fix ly:format for the string that contain zero

Fix ly:format for the string that contain zero

strcpy and strncpy cannot be used in std::string concatenation
because std::string may contain '\0' in its contents.

In order to avoid problems with string that contains '\0',
this commit replaces the strncpy to std::string::copy.

[dak, 2016-06-10 14:16:29]

https://codereview.appspot.com/300930043/diff/1/lily/general-scheme.cc
File lily/general-scheme.cc (left):

https://codereview.appspot.com/300930043/diff/1/lily/general-scheme.cc#oldcod...
lily/general-scheme.cc:598: strncpy (ptr, results[i].c_str (), results[i].size
());
I've suspected this one as well (basically I searched for "strn" and "stringn"
in the sources and this one was sort-of most suspicious) but I don't see where
ly:format would actually be called with the font data.

What did I overlook?

[trueroad, 2016-06-10 16:30:18]

A font embedding is no problem.
An EPS embedding was the problem.

`aaa-eps.eps` contains `\0` because it is embedded an OTF font.
Again, it is no problem.

`aaa-main.ly` uses `\epsfile`.
`\epsfile` had the problem.

`\epsfile` uses `ly:format` for EPS embedding.
In the case of the EPS which contains `\0`, `strncpy` in `ly:format` did not
copy subsequent of `\0`.

[trueroad, 2016-06-15 15:14:18]

I've pushed to staging.

Issue 4890: Fix ly:format for the string that contain zero
commit 211634be6f90db700eb8e5725046b27aa153ee92