logging: don't crash on nil strings

lib/pure/logging.nim

@@ -107,9 +107,14 @@ var
 proc substituteLog*(frmt: string, level: Level, args: varargs[string, `$`]): string =
   ## Format a log message using the ``frmt`` format string, ``level`` and varargs.
   ## See the module documentation for the format string syntax.
+  const nilString = "nil"
+
   var msgLen = 0
   for arg in args:
-    msgLen += arg.len
+    if arg.isNil:
+      msgLen += nilString.len
+    else:
+      msgLen += arg.len
   result = newStringOfCap(frmt.len + msgLen + 20)
   var i = 0
   while i < frmt.len:
@@ -136,7 +141,10 @@ proc substituteLog*(frmt: string, level: Level, args: varargs[string, `$`]): str
       of "levelname": result.add(LevelNames[level])
       else: discard
   for arg in args:
-    result.add(arg)
+    if arg.isNil:
+      result.add(nilString)
+    else:
+      result.add(arg)
 
 method log*(logger: Logger, level: Level, args: varargs[string, `$`]) {.
             raises: [Exception], gcsafe,
@@ -361,3 +369,6 @@ when not defined(testing) and isMainModule:
   addHandler(L)
   for i in 0 .. 25:
     info("hello", i)
+
+  var nilString: string
+  info "hello ", nilString