xspice/icm/digital/d_source/cfunc.mod, bug #301, increase MAX_STRING_SIZE for input file reading

Thats a workaround for a segmentation fault (buffer overrun)
  caused by too long lines in the input file for a xspice "d_source"

Thanks to Siddhant Saraf, who reported this in
  #301 d_source gives wrong error and then SIGSEGV (Address boundary error)
  http://sourceforge.net/p/ngspice/bugs/301/

src/xspice/icm/digital/d_source/cfunc.mod

@@ -61,7 +61,7 @@ NON-STANDARD FEATURES
 
 /*=== CONSTANTS ========================*/
 
-#define MAX_STRING_SIZE 200
+#define MAX_STRING_SIZE 1024
 
 
 /*=== MACROS ===========================*/