Fix arbitrary read from file system

If you pass path like `example.com//etc/passwd`, server will respond
with contents of `/etc/passwd` file

Signed-off-by: Alexey Yerin <yerinalexey98fd@gmail.com>

src/gemini.rs

@@ -14,13 +14,21 @@ impl GeminiRequest {
         Ok(gemini_request)
     }
 
-    pub fn file_path(&self) -> Option<&str> {
+    fn unsafe_file_path(&self) -> Option<&str> {
         self.path
             .path()
             .chars()
             .next()
             .map(|c| &self.path.path()[c.len_utf8()..])
     }
+
+    pub fn file_path(&self) -> Option<&str> {
+        match self.unsafe_file_path() {
+            Some(path) if path.contains("..") || path.starts_with("/") => None,
+            Some(path) => Some(path),
+            None => None,
+        }
+    }
 }
 
 fn parse_path(req: &str) -> Option<&str> {