Don't guess field widths unless we have at least 3 lines

- Investigation of #685 showed that single line files were being guessed
as fixed field width

main/src/com/google/refine/importers/FixedWidthImporter.java

@@ -185,7 +185,7 @@ public class FixedWidthImporter extends TabularImportingParserBase {
                     }
                 }
                 
-                if (counts != null) {
+                if (counts != null && lineCount > 2) {
                     List<Integer> widths = new ArrayList<Integer>();
                     
                     int startIndex = 0;