Fix OverflowError in _PyLong_FromByteArray.

Author: serhiy-storchaka

Objects/longobject.c

@@ -971,16 +971,9 @@ _PyLong_FromByteArray(const unsigned char* bytes, size_t n,
             ++numsignificantbytes;
     }
 
-    /* How many Python int digits do we need?  We have
-       8*numsignificantbytes bits, and each Python int digit has
-       PyLong_SHIFT bits, so it's the ceiling of the quotient. */
-    /* catch overflow before it happens */
-    if (numsignificantbytes > (PY_SSIZE_T_MAX - PyLong_SHIFT) / 8) {
-        PyErr_SetString(PyExc_OverflowError,
-                        "byte array too long to convert to int");
-        return NULL;
-    }
-    ndigits = (numsignificantbytes * 8 + PyLong_SHIFT - 1) / PyLong_SHIFT;
+    /* avoid integer overflow */
+    ndigits = numsignificantbytes / PyLong_SHIFT * 8
+        + (numsignificantbytes % PyLong_SHIFT * 8 + PyLong_SHIFT - 1) / PyLong_SHIFT;
     v = long_alloc(ndigits);
     if (v == NULL)
         return NULL;