Fix segfault in 1990/dds

In all versions the program would segfault if the file could not be
opened as it would write/close the NULL FILE *. Now it doesn't act on
the file unless f != NULL. If it is it just does the usual of writing
'Ok' to stdout.

1990/dds/README.md

@@ -16,7 +16,8 @@ you Yusuke!
 
 [Cody Boone Ferguson](/winners.html#Cody_Boone_Ferguson) removed the erroneous
 prototype to `fopen()` and made this use `fgets()` instead of `gets()` to make
-it safer. Thank you Cody for your assistance!
+it safer. He also fixed a segfault in the case that the file cannot be loaded.
+Thank you Cody for your assistance!
 
 ### To run
 
@@ -32,9 +33,6 @@ Give the following lines as input:
 
 NOTE: this is case sensitive.
 
-NOTE: this program will likely segfault if you specify a file that cannot be
-opened.
-
 ## Judges' comments: