better bubble up DB errors and guard one unwrap() where a crash occured during integration test run

(and possibly also related to #633)

src/dc_receive_imf.rs

@@ -610,7 +610,7 @@ unsafe fn add_parts(
     let icnt = mime_parser.parts.len();
     let mut txt_raw = None;
 
-    let is_ok = context
+    context
         .sql
         .prepare(
             "INSERT INTO msgs \
@@ -697,13 +697,10 @@ unsafe fn add_parts(
                 Ok(())
             },
         )
-        .is_ok();
-
-    if !is_ok {
-        // i/o error - there is nothing more we can do - in other cases, we try to write at least an empty record
-        cleanup(mime_in_reply_to, mime_references);
-        bail!("Cannot write DB.");
-    }
+        .map_err(|err| {
+            cleanup(mime_in_reply_to, mime_references);
+            err
+        })?;
 
     info!(
         context,

src/e2ee.rs

@@ -1,7 +1,6 @@
 //! End-to-end encryption support.
 
 use std::collections::HashSet;
-use std::ffi::CStr;
 use std::ptr;
 use std::str::FromStr;
 
@@ -479,8 +478,8 @@ fn update_gossip_peerstates(
             if !optional_field.fld_name.is_null()
                 && as_str(optional_field.fld_name) == "Autocrypt-Gossip"
             {
-                let value = unsafe { CStr::from_ptr(optional_field.fld_value).to_str().unwrap() };
-                let gossip_header = Aheader::from_str(value);
+                let value = to_string_lossy(optional_field.fld_value);
+                let gossip_header = Aheader::from_str(&value);
 
                 if let Ok(ref header) = gossip_header {
                     if recipients.is_none() {