Issue #17872: Fix a segfault in marshal.load() when input stream returns

more bytes than requested.

Lib/test/test_marshal.py

@@ -2,6 +2,7 @@
 
 from test import support
 import array
+import io
 import marshal
 import sys
 import unittest
@@ -279,6 +280,17 @@ class BugsTestCase(unittest.TestCase):
         unicode_string = 'T'
         self.assertRaises(TypeError, marshal.loads, unicode_string)
 
+    def test_bad_reader(self):
+        class BadReader(io.BytesIO):
+            def read(self, n=-1):
+                b = super().read(n)
+                if n is not None and n > 4:
+                    b += b' ' * 10**6
+                return b
+        for value in (1.0, 1j, b'0123456789', '0123456789'):
+            self.assertRaises(ValueError, marshal.load,
+                              BadReader(marshal.dumps(value)))
+
 LARGE_SIZE = 2**31
 pointer_size = 8 if sys.maxsize > 0xFFFFFFFF else 4