CHG: better way to handle file or version number

Author: MathieuMorlighem

src/m/modeldata/interpBedmachineGreenland.m

@@ -39,7 +39,7 @@
 
 basename = 'BedMachineGreenland';
 
-if nargin==5
+if nargin==5 && exist(ncdate, 'file')
 	ncfile = ncdate;
 else
 	%List of common paths to try