Add explanatory comment about fitting into a size_t.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4373)

crypto/evp/pbe_scrypt.c

@@ -207,6 +207,8 @@ int EVP_PBE_scrypt(const char *pass, size_t passlen,
 
     if (maxmem == 0)
         maxmem = SCRYPT_MAX_MEM;
+
+    /* Check that the maximum memory doesn't exceed a size_t limits */
     if (maxmem > SIZE_MAX)
         maxmem = SIZE_MAX;