[Bugs] #4826 Calculate UNSP: Incorrect output when solving a negative factorial

[Sugar Labs Bugs]

#4826: Incorrect output when solving a negative factorial
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       Reporter:  sarahmackey                |       Owner:  sarahmackey
           Type:  defect                     |      Status:  new
       Priority:  Unspecified by Maintainer  |   Milestone:  Unspecified
      Component:  Calculate                  |     Version:  Unspecified
       Severity:  Minor                      |    Keywords:
Distribution/OS:  Fedora                     |  Bug Status:  New
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 When the user inputs a negative integer within the factorial function the
 result is just the number that is put into the function.  The calculator
 prints it out and it appears that is the correct answer.  However, the
 answer should not be defined because there is no factorial value of a
 negative integer.

 for example:
     input: fac(-5)
     expected output: error or none
     actual output: -5.00000000

 Version: Using VirtualBox (firmware), Fedora Release 20 Heisenbug (Build),
 Sugar 0.103.2, Ubuntu 64-bit v.14.04.1
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Ticket URL: <http://bugs.sugarlabs.org/ticket/4826>
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