Derivative function panics when a bucket has one point only

[jvshahid]

With the following data in foo

[{"a": 2, "b": 1}, {"a": 2, "b": 15}, {"a": 2, "b": 15}]

Running a query with derivative like this

Influx> select count(a) from foo group by time(10s)
┌───────────────┬───────┐
│ time          │ count │
├───────────────┼───────┤
│ 1386262520000 │ 3     │
│ 1386262500000 │ 1     │
└───────────────┴───────┘
Influx> select derivative(a) from foo group by time(10s)

Result in the following panic:

********************************BUG********************************
Database: foo
Query: [select derivative(a) from foo group by time(1s)]
Error: runtime error: invalid memory address or nil pointer dereference. Stacktrace: goroutine 17 [running]:
engine.func·004()
    /home/jvshahid/codez/influxdb/src/engine/engine.go:29 +0x118
runtime.panic(0x7dd2a0, 0xe7c1e8)
    /home/jvshahid/.gvm/gos/go1.2/src/pkg/runtime/panic.c:248 +0x106
engine.(*DerivativeAggregator).GetValues(0xc2100bed40, 0xc2100fc478, 0x3, 0x7397a0, 0x4eccc659b4240, ...)
    /home/jvshahid/codez/influxdb/src/engine/aggregator.go:229 +0x11a
engine.(*QueryEngine).executeCountQueryWithGroupBy(0xc21000ad30, 0x7f26a06e17b8, 0xc2100db8c0, 0xc210076086, 0x3, ...)
    /home/jvshahid/codez/influxdb/src/engine/engine.go:354 +0xf98
engine.(*QueryEngine).RunQuery(0xc21000ad30, 0x7f26a06e17b8, 0xc2100db8c0, 0xc210076086, 0x3, ...)
    /home/jvshahid/codez/influxdb/src/engine/engine.go:43 +0x147
api/http.func·001(0x7f26a06e17b8, 0xc2100db8c0, 0x1, 0xc21005b640, 0x623538)
    /home/jvshahid/codez/influxdb/src/api/http/api.go:222 +0x28e
api/http.yieldUser(0x7f26a06e17b8, 0xc2100db8c0, 0x7f269c587c18, 0xc21007608c, 0x3, ...)
    /home/jvshahid/codez/influxdb/src/api/http/api.go:507 +0x3b
api/http.(*Ht