avoid potential null deref in as_cpp

[kevinushey]

Looking here:

cpp11/inst/include/cpp11/as.hpp

Lines 109 to 115 in cf7391b

	if (Rf_isString(from)) {
	if (Rf_xlength(from) == 1) {
	const char* c_p = nullptr;
	unwind_protect([&] { c_p = Rf_translateCharUTF8(STRING_ELT(from, 0)); });
	return c_p[0];
	}
	}

If an error occurs during translation, c_p will remain as a nullptr and then the attempt to access the zero-th element will likely crash.

[jimhester]

If an R error occurs in L112 in R versions with R_UnwindProtect() then unwind_protect() will throw a C++ exception, so L113 wouldn't run in that case. In R versions without R_UwindProtect() the code will cause an R error and again I believe L113 would not run, so I think it is actually OK as is.