Continue idmap generation even if name lookup fails.

In resources.arsc files, a resource is represented by a specification
block and one or more value blocks. In rare cases, a resource name
is also given a new resource ID, a specification block and no values
blocks. This commit ensures idmap generation does not fail if such an
entry is encountered.

Change-Id: I32302a0b07a7a320b7eeb31886931be3bb7b7e9a