Fixed incorrect reliance on errno in emubd

When running the tests, the emubd erase function relied on the value of
errno to not change over a possible call to unlink. Annoyingly, I've
only seen this cause problems on a couple of specific Travis instances
while self-hosting littlefs on top of littlefs-fuse.

emubd/lfs_emubd.c

@@ -196,7 +196,7 @@ int lfs_emubd_erase(const struct lfs_config *cfg, lfs_block_t block) {
         }
     }
 
-    if (errno == ENOENT || (S_ISREG(st.st_mode) && (S_IWUSR & st.st_mode))) {
+    if (err || (S_ISREG(st.st_mode) && (S_IWUSR & st.st_mode))) {
         FILE *f = fopen(emu->path, "w");
         if (!f) {
             return -errno;