No functional change. Add comment and assert to describe why there cannot be...

No functional change.  Add comment and assert to describe why there cannot be overflow which was reported by Klocwork.  Discussed on python-dev

Modules/unicodedata.c

@@ -395,6 +395,7 @@ unicodedata_decomposition(PyObject *self, PyObject *args)
     PyUnicodeObject *v;
     char decomp[256];
     int code, index, count, i;
+    unsigned int prefix_index;
 
     if (!PyArg_ParseTuple(args, "O!:decomposition",
 			  &PyUnicode_Type, &v))
@@ -428,9 +429,15 @@ unicodedata_decomposition(PyObject *self, PyObject *args)
     /* XXX: could allocate the PyString up front instead
        (strlen(prefix) + 5 * count + 1 bytes) */
 
+    /* Based on how index is calculated above and decomp_data is generated
+       from Tools/unicode/makeunicodedata.py, it should not be possible
+       to overflow decomp_prefix. */
+    prefix_index = decomp_data[index] & 255;
+    assert(prefix_index < (sizeof(decomp_prefix)/sizeof(*decomp_prefix)));
+
     /* copy prefix */
-    i = strlen(decomp_prefix[decomp_data[index] & 255]);
-    memcpy(decomp, decomp_prefix[decomp_data[index] & 255], i);
+    i = strlen(decomp_prefix[prefix_index]);
+    memcpy(decomp, decomp_prefix[prefix_index], i);
 
     while (count-- > 0) {
         if (i)