fix potential protect errors

[kevinushey]

From https://github.com/kalibera/rprotect:

maacheck

This tool is quite simple and only reports instances of multiple allocating arguments pattern, such as (from Rcpp code):

Rf_lang2(::Rf_install("simpleError"), Rf_mkString(str.c_str()))

This is a real error: Rf_mkString returns an unprotected newly allocated object. Rf_install may allocate and may be called after Rf_mkString, and hence may trigger garbage collection that will corrupt the object allocated by Rf_mkString.

The associated fix needs to occur here:

Rcpp/inst/include/Rcpp/exceptions.h

Line 294 in e81493b

Rcpp::Shield<SEXP> simpleErrorExpr( Rf_lang2(::Rf_install("simpleError"), Rf_mkString(str.c_str())) );

[eddelbuettel]

Also see #665