fix for issue 3862: test_array fails FreeBSD 7 amd64

FreeBSD 7's underlying malloc() is behaves differently to earlier versions
and seriously overcommits available memory on amd64.  This may affect
other 64bit platforms in some circumstances, so the scale of the
problematic test is wound back.

Patch by Mark Dickinson, reviewed by Martin von Loewis.

Lib/test/test_array.py

@@ -1011,20 +1011,21 @@ class DoubleTest(FPTest):
     minitemsize = 8
 
     def test_alloc_overflow(self):
+        from sys import maxsize
         a = array.array('d', [-1]*65536)
         try:
-            a *= 65536
+            a *= maxsize//65536 + 1
         except MemoryError:
             pass
         else:
-            self.fail("a *= 2**16 didn't raise MemoryError")
+            self.fail("Array of size > maxsize created - MemoryError expected")
         b = array.array('d', [ 2.71828183, 3.14159265, -1])
         try:
-            b * 1431655766
+            b * (maxsize//3 + 1)
         except MemoryError:
             pass
         else:
-            self.fail("a * 1431655766 didn't raise MemoryError")
+            self.fail("Array of size > maxsize created - MemoryError expected")
 
 tests.append(DoubleTest)