🚀 go-pugleaf

Problem Description:

Assuming an IMG input file with the name "gbbb.img", imageto dumps core
with a segmentation fault given the arguments:

    % imageto gbbb

(e.g. accidently omitting the ".img" extension).

Cursory investigation reveals that in the routine "set_input_format" in
"imageto/main.c" is not checking for a NULL return value from "find_suffix"
before doing STREQ comparisons on the result.  

Suggested fix:

   imageto/main.c:
   998c998
   <   if (STREQ (input_extension, "img"))
   ---
   >   if (input_extension && STREQ (input_extension, "img"))
   1001c1001
   <   else if (STREQ (input_extension, "pbm"))
   ---
   >   else if (input_extension && STREQ (input_extension, "pbm"))