guard against permission denied errors

reading from /proc/<pid>/smaps is not available inside of Docker container

script/memstats.rb

@@ -86,11 +86,18 @@ def consume_mapping( map_lines, totals )
   return m
 end
 
+def create_memstats_not_available( totals )
+  Mapping::FIELDS.each do |field|
+    totals[field] += Float::NAN
+  end
+end
+
 abort 'usage: memstats [pid]' unless ARGV.first
 pid = ARGV.shift.to_i
 totals = Hash.new(0)
 mappings = []
 
+begin
   File.open( "/proc/#{pid}/smaps" ) do |smaps|
 
     map_lines = []
@@ -112,6 +119,9 @@ File.open( "/proc/#{pid}/smaps" ) do |smaps|
       end
     end
   end
+rescue
+  create_memstats_not_available( totals )
+end
 
 # http://rubyforge.org/snippet/download.php?type=snippet&id=511
 def format_number( n )