[LIBS] vfs: Fix usage of strrchr()

Recent changes in stringlib made semantics of strrchr()
UNIX compatible.

According to this strrchr() would return NULL when intended
character is not found in the string.

This patch fixes vfs code as per updated UNIX compatible
semantics of strrchr()

Signed-off-by: Anup Patel <anup@brainfault.org>