Do not rely on ArrayIndexOutOfBoundsException to detect end of input

Do not rely on ArrayIndexOutOfBoundsException to detect end of input

In the Config#StringReader we relied on ArrayIndexOutOfBoundsException
to detect the end of the input. Creation of exception with (deep) stack
trace can significantly degrade performance in case when we read
thousands of config files, like in the case when Gerrit reads all
external ids from the NoteDb.

Use the buf.length to detect the end of the input.

Change-Id: I12266f25751373a870ce3fa623cf2a95d882d521