Fix crashes with invalid utf-8.

According to the discussion in emacs bug#74922 it's possible that
emacs passes invalid strings to dynamic libraries.

Author: kurnevsky

Cargo.toml

@@ -34,7 +34,7 @@ emacs-macros = { path = "emacs-macros", version = "0.17.0" }
 rustc_version = "0.2.3"
 
 [features]
-default = []
+default = [ "utf-8-validation" ]
 utf-8-validation = []
 lossy-integer-conversion = []
 nonzero-integer-conversion = []

src/types/string.rs

@@ -13,7 +13,7 @@ impl FromLisp<'_> for String {
     #[cfg(feature = "utf-8-validation")]
     fn from_lisp(value: Value<'_>) -> Result<Self> {
         let bytes = value.env.string_bytes(value)?;
-        Ok(String::from_utf8(bytes).unwrap())
+        String::from_utf8(bytes).map_err(|e| e.into())
     }
 }