Avoid a segmentation fault with renaming merges

Under very particular circumstances, merge-recursive's `add_cacheinfo()`
function gets a `NULL` returned from `refresh_cache_entry()` without
expecting it, and subsequently passes it to `add_cache_entry()` which
consequently crashes.

Let's not crash.

This fixes #952

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

merge-recursive.c

@@ -235,9 +235,7 @@ static int add_cacheinfo(struct merge_options *o,
 		struct cache_entry *nce;
 
 		nce = refresh_cache_entry(ce, CE_MATCH_REFRESH | CE_MATCH_IGNORE_MISSING);
-		if (!nce)
-			return err(o, _("addinfo_cache failed for path '%s'"), path);
-		if (nce != ce)
+		if (nce && nce != ce)
 			ret = add_cache_entry(nce, options);
 	}
 	return ret;