Fix 2018/algmyr to not segfault on NULL file

Now it just exits 1 if a file cannot be opened. I do have a concern with
this type of fix which I will bring up on GitHub but it seems better
anyway to not crash with invalid input unless the entry should crash.
The fact the author noted that invalid paths as a limitation suggests in
this case that it is reasonable to fix. There are other limitations /
bugs the author mentions but perhaps those should not be fixed. That
remains to be seen I guess.

I can also now confirm that this entry does indeed work as expected; I
could not test it with commit ced8177ef9
but I had to test it here to make sure the changes did not affect the
functionality.

The original source, which has a really nice layout, can be found in
prog.alt.c and is referenced in the README.md. It can be built by make
alt.

2018/algmyr/README.md

@@ -17,6 +17,12 @@ cat prog.c | ./prog           # Printing garbage might break your font
 ./prog -d 2 0 out.raw         # Decode the first (0th) channel out of two
 ```
 
+[Cody Boone Ferguson](/winners.html#Cody_Boone_Ferguson) made this entry not
+crash when a file cannot be opened. The original file, kept for the wonderful
+format, is in [prog.alt.c](prog.alt.c). See below for details. Thank you Cody
+for your assistance!
+
+
 ## Try:
 
 ```sh
@@ -25,15 +31,29 @@ cat prog.c | ./prog           # Printing garbage might break your font
 # Who needs cat?
 ./prog prog.c | ./prog -d 1 0
 
-# Assuming ALSA
+# Assuming ALSA:
 echo '<<<<<<  /\_/\_/\  _-_-_-_  !!!!!  :.:.:.:  >>>>>> ****** ~~~~~~~' |
     ./prog | aplay -c1 -fFLOAT_LE -r44100
 
-# Assuming sox
+# Assuming sox:
 echo -n ' MENE MENE TEKEL UPHARSIN ' | ./prog | 
     sox -t raw -c 1 -r 44100 -L -e floating-point -b 32 - -n spectrogram -d 10 -X 300
+
+# now open spectrogram.png with a graphics viewer or editor
 ```
 
+### Alternate code:
+
+The original source which will crash if a file cannot be opened but which also
+has a nicer layout can be found in [prog.alt.c](prog.alt.c). Try:
+
+
+```sh
+make alt
+```
+
+Use `prog.alt` as you would `prog`.
+
 ## Judges' comments:
 
 Be warned, this is not your garden variety text-to-speech program! Earplugs are advised,