STORE: simplify store_loader_cmp()

We have already made sure that the loader scheme isn't NULL, so
checking if they are NULL or not when comparing registered loaders
is redundant.  We still soft assert it, just to be entirely sure.
Reviewed-by: NBen Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/3805)

crypto/store/store_register.c

@@ -123,11 +123,8 @@ static unsigned long store_loader_hash(const OSSL_STORE_LOADER *v)
 static int store_loader_cmp(const OSSL_STORE_LOADER *a,
                             const OSSL_STORE_LOADER *b)
 {
-    if (a->scheme != NULL && b->scheme != NULL)
-        return strcmp(a->scheme, b->scheme);
-    else if (a->scheme == b->scheme)
-        return 0;
-    return a->scheme == NULL ? -1 : 1;
+    assert(a->scheme != NULL && b->scheme != NULL);
+    return strcmp(a->scheme, b->scheme);
 }
 
 static LHASH_OF(OSSL_STORE_LOADER) *loader_register = NULL;