cleanup: Ensure message structs don't have uninitialised members.

Coverity tripped over this. In reality this is probably fine, but the
cost of this extra initialisation is negligible.

src/model/ichatlog.h

@@ -26,10 +26,10 @@ Q_DECLARE_METATYPE(ChatLogIdx)
 struct SearchPos
 {
     // Index to the chat log item we want
-    ChatLogIdx logIdx;
+    ChatLogIdx logIdx{0};
     // Number of matches we've had. This is always number of matches from the
     // start even if we're searching backwards.
-    size_t numMatches;
+    size_t numMatches{0};
 
     bool operator==(const SearchPos& other) const
     {
@@ -54,10 +54,10 @@ struct SearchPos
 
 struct SearchResult
 {
-    bool found;
+    bool found{false};
     SearchPos pos;
-    size_t start;
-    size_t len;
+    size_t start{0};
+    size_t len{0};
 
     // This is unfortunately needed to shoehorn our API into the highlighting
     // API of above classes. They expect to re-search the same thing we did

src/model/message.h

@@ -26,14 +26,14 @@ struct MessageMetadata
 {
     MessageMetadataType type;
     // Indicates start position within a Message::content
-    size_t start;
+    size_t start{0};
     // Indicates end position within a Message::content
-    size_t end;
+    size_t end{0};
 };
 
 struct Message
 {
-    bool isAction;
+    bool isAction{false};
     QString content;
     QDateTime timestamp;
     std::vector<MessageMetadata> metadata;