src/__init__.py tests/: avoid segv from fz_samples_get() with empty p…

…ixmap.

Pixmap.color_count(): don't raise exception if JM_color_count() returns empty
dict.

_read_samples(): return empty list if pixmap has no samples - avoids segv from
fz_samples_get().

Addresses #3848.

src/__init__.py

@@ -10171,8 +10171,6 @@ def color_count(self, colors=0, clip=None):
         '''
         pm = self.this
         rc = JM_color_count( pm, clip)
-        if not rc:
-            raise RuntimeError( MSG_COLOR_COUNT_FAILED)
         if not colors:
             return len( rc)
         return rc
@@ -14219,6 +14217,9 @@ def _read_samples( pixmap, offset, n):
     # fixme: need to be able to get a sample in one call, as a Python
     # bytes or similar.
     ret = []
+    if not pixmap.samples():
+        # mupdf.fz_samples_get() gives a segv if pixmap->samples is null.
+        return ret
     for i in range( n):
         ret.append( mupdf.fz_samples_get( pixmap, offset + i))
     return bytes( ret)

tests/test_pixmap.py

@@ -363,3 +363,16 @@ def do(gi):
     else:
         assert out1 != out0
     assert out2 == out0
+
+
+def test_3848():
+    path = os.path.normpath(f'{__file__}/../../tests/resources/test_3848.pdf')
+    with pymupdf.open(path) as document:
+        for i in range(len(document)):
+            page = document.load_page(i)
+            print(f'{page=}.')
+            for annot in page.get_drawings():
+                if page.get_textbox(annot['rect']):
+                    rect = annot['rect']
+                    pixmap = page.get_pixmap(clip=rect)
+                    color_bytes = pixmap.color_topusage()