CORE: Fix small bug in passphrase caching

Passphrase caching didn't allocate memory when it got to cache an
empty string, leading to a crash.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12587)

crypto/passphrase.c

@@ -273,7 +273,8 @@ int ossl_pw_get_passphrase(char *pass, size_t pass_size, size_t *pass_len,
 
  do_cache:
     if (ret && data->flag_cache_passphrase) {
-        if (*pass_len > data->cached_passphrase_len) {
+        if (data->cached_passphrase == NULL
+            || *pass_len > data->cached_passphrase_len) {
             void *new_cache =
                 OPENSSL_clear_realloc(data->cached_passphrase,
                                       data->cached_passphrase_len,