Fix compiler warnings with openssl-scrypt/libscrypt test on clang

Clang didn't like that we were passing uint64_t values to an API
that wanted uint32_t.  GCC has either not cared, or has figured out
that the values in question were safe to cast to uint32_t.

Fixes bug22916; bugfix on 0.2.7.2-alpha.

changes/bug22916_027

@@ -0,0 +1,3 @@
+  o Minor bugfixes (Compilation):
+    - Fix warnings when building with libscrypt and openssl scrypt support
+      on Clang. Fixes bug 22916; bugfix on 0.2.7.2-alpha.

src/test/test_crypto_slow.c

@@ -136,7 +136,8 @@ test_libscrypt_eq_openssl(void *arg)
   uint8_t buf1[64];
   uint8_t buf2[64];
 
-  uint64_t N, r, p;
+  uint64_t N;
+  uint32_t r, p;
   uint64_t maxmem = 0; // --> SCRYPT_MAX_MEM in OpenSSL.
 
   int libscrypt_retval, openssl_retval;