Make sure OPENSSL_cleanse checks for NULL

In master we have the function OPENSSL_clear_free(x,y), which immediately
returns if x == NULL. In <=1.0.2 this function does not exist so we have to
do:
OPENSSL_cleanse(x, y);
OPENSSL_free(x);

However, previously, OPENSSL_cleanse did not check that if x == NULL, so
the real equivalent check would have to be:
if (x != NULL)
    OPENSSL_cleanse(x, y);
OPENSSL_free(x);

It would be easy to get this wrong during cherry-picking to other branches
and therefore, for safety, it is best to just ensure OPENSSL_cleanse also
checks for NULL.
Reviewed-by: NRich Salz <rsalz@openssl.org>

crypto/mem_clr.c

@@ -66,6 +66,10 @@ void OPENSSL_cleanse(void *ptr, size_t len)
 {
     unsigned char *p = ptr;
     size_t loop = len, ctr = cleanse_ctr;
+
+    if (ptr == NULL)
+        return;
+
     while (loop--) {
         *(p++) = (unsigned char)ctr;
         ctr += (17 + ((size_t)p & 0xF));