fix(postgres): don't panic if M or C Notice fields are not UTF-8

This has been observed with an old version of PostgreSQL (11.0.4)
running on Windows Server 2016 with windows-1252 encoding and French
locale.

This change replaces invalid UTF-8 fields with a default string, so the
other fields can still be read if they are valid.

sqlx-postgres/src/message/response.rs

@@ -6,6 +6,11 @@ use sqlx_core::bytes::Bytes;
 use crate::error::Error;
 use crate::io::Decode;
 
+const INVALID_UTF8: &str = "Postgres returned a non-UTF-8 string for its error message. \
+         This is most likely due to an error that occurred during authentication and \
+         the default lc_messages locale is not binary-compatible with UTF-8. \
+         See the server logs for the error details.";
+
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
 #[repr(u8)]
 pub enum PgSeverity {
@@ -100,8 +105,7 @@ impl Notice {
 
     #[inline]
     fn get_cached_str(&self, cache: (u16, u16)) -> &str {
-        // unwrap: this cannot fail at this stage
-        from_utf8(&self.storage[cache.0 as usize..cache.1 as usize]).unwrap()
+        from_utf8(&self.storage[cache.0 as usize..cache.1 as usize]).unwrap_or(INVALID_UTF8)
     }
 }
 
@@ -203,13 +207,7 @@ impl<'a> Iterator for Fields<'a> {
 
 fn notice_protocol_err() -> Error {
     // https://github.com/launchbadge/sqlx/issues/1144
-    Error::Protocol(
-        "Postgres returned a non-UTF-8 string for its error message. \
-         This is most likely due to an error that occurred during authentication and \
-         the default lc_messages locale is not binary-compatible with UTF-8. \
-         See the server logs for the error details."
-            .into(),
-    )
+    Error::Protocol(INVALID_UTF8.into())
 }
 
 #[test]