Sanitize $nodeSourceID in browse

#33 identified that nodeValues could be malformed and sent to the
database. While they were escaped and it doesn’t appear SQL injection
is possible through this vector, the behavior was still odd.

This change validates the input and ignores if it is invalid.

Fixes #33
Merge pull request #61 from dleehr/fix-sql-injection

Sanitize $nodeSourceID in browse