Remove assert from is_partially_overlapping()

This function is used to validate application supplied parameters. An
assert should be used to check for an error that is internal to OpenSSL.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2275)

crypto/evp/evp_enc.c

@@ -286,7 +286,7 @@ int is_partially_overlapping(const void *ptr1, const void *ptr2, int len)
      */
     int overlapped = (len > 0) & (diff != 0) & ((diff < (PTRDIFF_T)len) |
                                                 (diff > (0 - (PTRDIFF_T)len)));
-    assert(!overlapped);
+
     return overlapped;
 }