[Couenne] .sol output fix

[victor.zverovich at gmail.com]

The attached patch fixes .sol file output for trivially infeasible problems
such as

  var x >= 0;
  s.t. c: x <= -1;

Currently trying to solve it with couenne gives an error:

  Error at _cmdno 4 executing "solve" command
  (file test.ampl, line 6, offset 84):
  Bad line in test.sol:  Couenne (test.nl May 18 2015): Infeasible

because the solution format is incorrect (extra blank line at the
beginning).

Best regards,
Victor
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