Fix bug when a line happens to parse as JSON but isn't an object.

For example, if `false` happened to be the only value on a line, this
would be parsed as valid JSON (a boolean) but it is not a structured log
message that slfmt should try to parse and format.

slfmt.nimble

@@ -1,6 +1,6 @@
 # Package
 
-version       = "0.2.0"
+version       = "0.2.1"
 author        = "Jonathan Bernard"
 description   = "Small utility to pretty-print strucutured logs."
 license       = "MIT"

src/slfmt.nim

@@ -4,7 +4,7 @@ import docopt, timeutils, zero_functional
 from std/logging import Level
 from std/sequtils import toSeq
 
-const VERSION = "0.2.0"
+const VERSION = "0.2.1"
 
 const USAGE = """Usage:
    slfmt [options]
@@ -134,6 +134,8 @@ when isMainModule:
     while(sin.readLine(line)):
       try:
         let logJson = parseLogLine(line)
+        if logJson.kind != JObject:
+          raise newException(JsonParsingError, "Expected a JSON object")
 
         if logLevel.isSome and logJson.hasKey("level"):
           let lvl = parseLogLevel(logJson["level"].getStr)