No check for metric value

[sstolpovskiy]

Hello,

I have faced the issue:

1. index is created using default metric in python.
2. index is read in java with Euclidean metric without error.

On search raised following error:
java.lang.ArrayIndexOutOfBoundsException: 2820
at com.spotify.annoy.ANNIndex.getIntInAnnBuf(ANNIndex.java:133) ~[annoy-0.2.6-SNAPSHOT.jar:na]
at com.spotify.annoy.ANNIndex.getNearest(ANNIndex.java:257) ~[annoy-0.2.6-SNAPSHOT.jar:na]

Where 2820 number depend so how on index size.

I think it should be some check for valid metric.