src/templ.rs: fix handling of C string slices

CommandLineParser is currently misbehaving due to improper data in
its input argv parameter. The problem is caused by a blind cast of
argv entries from &str into a C-like string, which does not offer the
null termination guarantees. To fix that, create an actual CString
and hold it while the pointer is in use.

It's worth noting that the mutable version of the macro still relies
on undefined behavior, but this time of CString which does not
guarantee correctness if its data is written into.

src/templ.rs

@@ -79,17 +79,26 @@ macro_rules! string_array_arg {
 	($name: ident) => {
 		let $name = $name
 			.iter()
-			.map(|x| x.as_ptr().cast::<::std::ffi::c_char>())
+			.map(|x| ::std::ffi::CString::new(*x).expect("invalid C string"))
+			.collect::<::std::vec::Vec<_>>();
+		let $name = $name
+			.iter()
+			.map(|x| x.as_ptr())
 			.collect::<::std::vec::Vec<_>>();
 	};
 }
 
 #[allow(unused_macros)]
 macro_rules! string_array_arg_mut {
 	($name: ident) => {
+		let $name = $name
+			.iter()
+			.map(|x| ::std::ffi::CString::new(*x).expect("invalid C string"))
+			.collect::<::std::vec::Vec<_>>();
+		// Casting to mut below trusts on undefined CString behavior.
 		let mut $name = $name
 			.iter()
-			.map(|x| x.as_ptr().cast::<::std::ffi::c_char>().cast_mut())
+			.map(|x| x.as_ptr().cast_mut())
 			.collect::<::std::vec::Vec<_>>();
 	};
 }