src/__init__.py tests/: fix Pixmap.is_unicolor segv if pixmap is empty.

This partially addresses #4466.

Author: julian-smith-artifex-com

src/__init__.py

@@ -10258,11 +10258,13 @@ def _pixmap_read_samples(pm, offset, n):
             for i in range(n):
                 ret.append(mupdf.fz_samples_get(pm, offset+i))
             return ret
-        sample0 = _pixmap_read_samples( pm, 0, n)
-        for offset in range( n, count, n):
-            sample = _pixmap_read_samples( pm, offset, n)
-            if sample != sample0:
-                return False
+        for offset in range( 0, count, n):
+            if offset == 0:
+                sample0 = _pixmap_read_samples( pm, 0, n)
+            else:
+                sample = _pixmap_read_samples( pm, offset, n)
+                if sample != sample0:
+                    return False
         return True
 
     @property

tests/test_4466.pdf

@@ -1685,3 +1685,12 @@ def test_4415():
         pixmap.save(path_out)
         rms = gentle_compare.pixmaps_rms(path_out_expected, path_out)
         assert rms == 0, f'{rms=}'
+
+def test_4466():
+    path = os.path.normpath(f'{__file__}/../../tests/test_4466.pdf')
+    with pymupdf.Document(path) as document:
+        for page in document:
+            print(f'{page=}', flush=1)
+            pixmap = page.get_pixmap(clip=(0, 0, 10, 10))
+            print(f'{pixmap.n=} {pixmap.size=} {pixmap.stride=} {pixmap.width=} {pixmap.height=} {pixmap.x=} {pixmap.y=}', flush=1)
+            pixmap.is_unicolor  # Used to crash.