Queries with median cause the server to panic and throw BUG errors in the log

[jvshahid]

Running select median(cpu) from test_medians group by host; cause the following error to be thrown (test median has three points 1 second apart for two hosts hosta and hostb):

Error: runtime error: invalid memory address or nil pointer dereference. Stacktrace: goroutine 22 [running]:
engine.func·004()
    /home/jvshahid/codez/influxdb/src/engine/engine.go:27 +0xfd
runtime.panic(0x7b0be0, 0xe181c8)
    /home/jvshahid/bin/go/src/pkg/runtime/panic.c:248 +0x106
engine.(*QueryEngine).executeCountQueryWithGroupBy(0xc210037b50, 0x7f46884b77b0, 0xc2100cc070, 0xc210059726, 0x3, ...)
    /home/jvshahid/codez/influxdb/src/engine/engine.go:321 +0x133c
engine.(*QueryEngine).RunQuery(0xc210037b50, 0x7f46884b77b0, 0xc2100cc070, 0xc210059726, 0x3, ...)
    /home/jvshahid/codez/influxdb/src/engine/engine.go:39 +0x117
api/http.func·001(0x7f46884b77b0, 0xc2100cc070, 0x5e7c68, 0xc21012da30, 0xc21012d9b0)
    /home/jvshahid/codez/influxdb/src/api/http/api.go:198 +0x2e2
api/http.yieldUser(0x7f46884b77b0, 0xc2100cc070, 0x7f46840c3c18, 0xc21005972c, 0x4, ...)
    /home/jvshahid/codez/influxdb/src/api/http/api.go:465 +0x3b
api/http.(*HttpServer).tryAsDbUser(0xc2100593c0, 0x7f46884b74a0, 0xc2100ce500, 0xc21011e4e0, 0x7f46840c3c18, ...)
    /home/jvshahid/codez/influxdb/src/api/http/api.go:619 +0x291
api/http.