Improved PcxImagePlugin test coverage

Tests/test_file_pcx.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import io
 from pathlib import Path
 
 import pytest
@@ -36,6 +37,28 @@
         im.save(f)
 
 
+def test_bad_image_size() -> None:
+    with open("Tests/images/pil184.pcx", "rb") as fp:
+        data = fp.read()
+        data = data[:4] + b"\xff\xff" + data[6:]
+
+    b = io.BytesIO(data)
+    with pytest.raises(SyntaxError, match="bad PCX image size"):
+        with PcxImagePlugin.PcxImageFile(b):
+            pass
+
+
+def test_unknown_mode() -> None:
+    with open("Tests/images/pil184.pcx", "rb") as fp:
+        data = fp.read()
+        data = data[:3] + b"\xff" + data[4:]
+
+    b = io.BytesIO(data)
+    with pytest.raises(OSError, match="unknown PCX mode"):
+        with Image.open(b):
+            pass
+
+
 def test_invalid_file() -> None:
     invalid_file = "Tests/images/flower.jpg"