smtp/mime: consolidate a MimeStateSMTP restart function

Ticket: 8649

Fully resets all the fields before tackling an ecapsulated message
to avoid evasion, due to the encoding of the upper file
leaking into the next one...

(cherry picked from commit 4985eb9dae)

rust/src/mime/smtp.rs

@@ -95,6 +95,19 @@ pub struct MimeStateSMTP<'a> {
     pub(crate) md5_result: GenericArray<u8, U16>,
 }
 
+impl MimeStateSMTP<'_> {
+    fn restart(&mut self) {
+        self.state_flag = MimeSmtpParserState::MimeSmtpStart;
+        self.headers.truncate(self.main_headers_nb);
+        self.encoding = MimeSmtpEncoding::Plain;
+        self.decoder = None;
+        self.filename.clear();
+        self.bufeolen = 0;
+        self.content_type = MimeSmtpContentType::Message;
+        self.decoded_line.clear();
+    }
+}
+
 pub fn mime_smtp_state_init(
     files: &mut FileContainer, sbcfg: *const StreamingBufferConfig,
 ) -> Option<MimeStateSMTP<'_>> {
@@ -392,8 +405,8 @@ fn mime_smtp_parse_line(
                     ctx.main_headers_nb = ctx.headers.len();
                 }
                 if encap_msg {
-                    ctx.state_flag = MimeSmtpParserState::MimeSmtpStart;
-                    ctx.headers.truncate(ctx.main_headers_nb);
+                    // looks like we have 0 headers...
+                    ctx.restart();
                     return (MimeSmtpParserResult::MimeSmtpNeedsMore, warnings);
                 }
                 ctx.state_flag = MimeSmtpParserState::MimeSmtpBody;
@@ -420,8 +433,7 @@ fn mime_smtp_parse_line(
                     ctx.main_headers_nb = ctx.headers.len();
                 }
                 if encap_msg {
-                    ctx.state_flag = MimeSmtpParserState::MimeSmtpStart;
-                    ctx.headers.truncate(ctx.main_headers_nb);
+                    ctx.restart();
                     return (MimeSmtpParserResult::MimeSmtpNeedsMore, warnings);
                 }
                 ctx.state_flag = MimeSmtpParserState::MimeSmtpBody;
@@ -475,15 +487,11 @@ fn mime_smtp_parse_line(
                             }
                         }
                     }
-                    ctx.state_flag = MimeSmtpParserState::MimeSmtpStart;
                     let toclose = !ctx.filename.is_empty();
-                    ctx.filename.clear();
-                    ctx.headers.truncate(ctx.main_headers_nb);
-                    ctx.encoding = MimeSmtpEncoding::Plain;
-                    ctx.bufeolen = 0;
                     if i.len() >= b.len() + 2 && i[b.len()] == b'-' && i[b.len() + 1] == b'-' {
                         ctx.boundaries.pop();
                     }
+                    ctx.restart();
                     if toclose {
                         return (MimeSmtpParserResult::MimeSmtpFileClose, 0);
                     }