update megaraid_serial fact to deal with SM motherboards

Per jhoblitt#29, the LSI controller on at least some models of SuperMicro
motherboards does not report a serial number.  The megaraid_serial fact
has not gracefully handling this case and facter was generating errors.

lib/facter/megaraid_serial.rb

@@ -6,6 +6,7 @@
       output = Facter::Util::Resolution.exec("#{megacli} -Version -Ctrl -aALL -NoLog")
       next if output.nil?
       m = output.match(/Serial No : (\S+)\s*$/)
+      next if m.nil?
       next unless m.size == 2
       m[1]
     end

spec/unit/facts/megaraid_serial_spec.rb

@@ -25,6 +25,16 @@
         returns(File.read(fixtures('megacli', 'version-ctrl-aall-8.07.07')))
       Facter.fact(:megaraid_serial).value.should == 'SV22925366'
     end
+
+    context 'megacli output is missing serial number' do
+      it 'should get the version string' do
+        Facter.fact(:megacli).stubs(:value).returns('/usr/bin/MegaCli')
+        Facter::Util::Resolution.stubs(:exec).
+          with('/usr/bin/MegaCli -Version -Ctrl -aALL -NoLog').
+          returns(File.read(fixtures('megacli', 'version-ctrl-aall-sm_no_serial')))
+        Facter.fact(:megaraid_serial).value.should be_nil
+      end
+    end
   end
 
 end