Some codeql fixes

src/qt/qt_platform.cpp

@@ -341,9 +341,25 @@ path_get_slash(char *path)
 void
 path_append_filename(char *dest, const char *s1, const char *s2)
 {
-    strcpy(dest, s1);
-    path_slash(dest);
-    strcat(dest, s2);
+    size_t dest_size = 260;
+    size_t len;
+
+    if (!dest || !s1 || !s2)
+        return;
+
+    snprintf(dest, dest_size, "%s", s1);
+    len = strlen(dest);
+
+    if (len > 0 && dest[len - 1] != '/' && dest[len - 1] != '\\') {
+        if (len + 1 < dest_size) {
+            dest[len++] = '/';
+            dest[len] = '\0';
+        }
+    }
+
+    if (len < dest_size - 1) {
+        strncat(dest, s2, dest_size - len - 1);
+    }
 }
 
 void