avoid potentially unsafe Ruby C API usage w/o GVL
avoid potentially unsafe Ruby C API usage w/o GVL

Using StringValuePtr() /can/ potentially allocate a new String
object.  This allocation _currently_ impossible in our codebase,
but to prevent future/potential bugs from appearing, we'll
avoid StringValuePtr() (and any functions not explicitly
supported w/o GVL) in nogvl_* regions.

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