fix bug with firstobservedperiod. closes #18

(cherry picked from commit 6b17c3de353d7202d4b47b7cc437063f4d542ad3)

src/@dseries/firstobservedperiod.m

@@ -8,7 +8,7 @@ function d = firstobservedperiod(o) % --*-- Unitary tests --*--
 % OUTPUTS 
 % - b [dates]      First period where the N variables are observed (without NaNs).
 
-% Copyright (C) 2016 Dynare Team
+% Copyright (C) 2016-2017 Dynare Team
 %
 % This file is part of Dynare.
 %
@@ -27,6 +27,9 @@ function d = firstobservedperiod(o) % --*-- Unitary tests --*--
 
 b = ~isnan(o);
 c = find(prod(b, 2));
+if isempty(c)
+    error('No overlapping non-NaN data points found in dseries.');
+end
 d = firstdate(o)+(c(1)-1);
 
 %@test:1