Panic when using a where clause that reference columns with null values

[jvshahid]

With the following data in foo

[
  {
    "name": "test_issue_105",
    "columns": ["time", "a", "b"],
    "points":[
          [1386262529794, 2, 1],
          [1386262529794, 2, null]
      ]
  }
]

Running the queries

Influx> select * from foo where b > 0
Influx> select * from foo where b > 0.0

will cause the following panic and no results to be returned

********************************BUG********************************
Database: foo
Query: [select * from foo where b > 0]
Error: runtime error: invalid memory address or nil pointer dereference. Stacktrace: goroutine 62 [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
common.CoerceValues(0x0, 0xc2100db880, 0xc2100db880, 0x0, 0xc2100c57b0, ...)
    /home/jvshahid/codez/influxdb/src/common/coerce_values.go:18 +0x28
datastore.GreaterThanOperator(0x0, 0xc2100db880, 0xc210074170, 0xc210074100, 0xc2100740d0)
    /home/jvshahid/codez/influxdb/src/datastore/boolean_operators.go:92 +0x31
datastore.func·001(0x0, 0xc2100cfcc0, 0x1, 0x1, 0xc210074170, ...)
    /home/jvshahid/codez/influxdb/src/datastore/boolean_operators.go:19 +0x98
datastore.matchesExpression(0xc2100db100, 0xc21004c900, 0x2, 0x2, 0xc2100db6c0, ...)
    /home/jvshahid/codez/influxdb/src/datastore/filtering.go:60 +0x23d
datastore.matches(0xc210120f30, 0xc21004c900, 0x2, 0x2, 0xc2100db6c0, ...)
    /home/jvshahid/codez/influxdb/src/datastore/filtering.go:65 +0x91
datastore.Filter(0xc21014a090, 0xc2100d1910, 0x3, 0xc2100c5ad0, 0x1)