src: use .data() instead of .c_str()
Since C++11 onwards, .data() is required to be null terminated, and can be used always instead of .c_str().
I mildly prefer c_str(), but if Node only supports C++11, this will convert to .data().
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines
Affected core subsystem(s)
src
cf. https://github.com/nodejs/node/pull/11006#discussion_r100238194