MozNSS Compat. Layer: fix incorrect parsing of CACertDir
NSS DB type prefix was not taken into account at all. Due to this the
path might not have been stat-ed. Thus, last part of the path would
have been considered an NSS DB name prefix which would be incorrect.
(cherry picked from commit 7f41b4a1ffe61c03d65896d82fc6b72a2710c492)