Fix a bug that deleted too many hosts

When running a command that takes host ranges as arguments (e.g.,
`hosts`, `services`), the arguments get parsed by
Rex::Socket::RangeWalker. If RangeWalker was unable to parse, it would
return nil, which in this context means "all hosts." If the user is
searching, they get all hosts instead of the ones they were interested
in -- this is annoying, but not too big a deal. Unfortunately, the same
logic applied when *deleting* hosts, with `hosts -d ...`, causing all
hosts to be deleted when giving it an invalid range.

lib/msf/ui/console/command_dispatcher/db.rb

@@ -1745,23 +1745,31 @@ class Db
   # Miscellaneous option helpers
   #
 
+  # Parse +arg+ into a {RangeWalker} and append the result into +host_ranges+
   #
-  # Parse +arg+ into a RangeWalker and append the result into +host_ranges+
-  #
-  # Returns true if parsing was successful or nil otherwise.
-  #
-  # NOTE: This modifies +host_ranges+
+  # @note This modifies +host_ranges+ in place
   #
+  # @param arg [String] The thing to turn into a RangeWalker
+  # @param host_ranges [Array] The array of ranges to append
+  # @param required [Boolean] Whether an empty +arg+ should be an error
+  # @return [Boolean] true if parsing was successful or nil otherwise
   def arg_host_range(arg, host_ranges, required=false)
     if (!arg and required)
       print_error("Missing required host argument")
-      return
+      return false
     end
     begin
-      host_ranges << Rex::Socket::RangeWalker.new(arg)
+      rw = Rex::Socket::RangeWalker.new(arg)
     rescue
       print_error("Invalid host parameter, #{arg}.")
-      return
+      return false
+    end
+
+    if rw.valid?
+      host_ranges << rw
+    else
+      print_error("Invalid host parameter, #{arg}.")
+      return false
     end
     return true
   end