Correct function type of snprintf

man snprintf:

Upon successful return, these functions return the number of characters printed (excluding the null byte used to end output to strings).

The functions snprintf() and vsnprintf() do not write more than size bytes (including the terminating null byte ('\0')).  If the output  was
truncated  due  to this limit, then the return value is the number of characters (excluding the terminating null byte) which would have been
written to the final string if enough space had been available.  Thus, a return value of size or more means that the output  was  truncated.
(See also below under NOTES.)

If an output error is encountered, a negative value is returned.

Signed-off-by: Mikko Koivunalho <mikko.koivunalho@iki.fi>

tests/check_check_master.c

@@ -934,7 +934,7 @@ void record_failure_line_num(int linenum)
 {
   size_t to_write;
   size_t written;
-  int result;
+  int result, chars_printed;
   char string[16];
 
   /*
@@ -943,12 +943,13 @@ void record_failure_line_num(int linenum)
    */
    linenum += 1;
 
-  to_write = snprintf(string, sizeof(string), "%d\n", linenum);
-  if(to_write == 0)
+  chars_printed = snprintf(string, sizeof(string), "%d\n", linenum);
+  if(chars_printed <= 0 || (size_t) chars_printed >= sizeof(string))
   {
     fprintf(stderr, "%s:%d: Error in call to snprintf:", __FILE__, __LINE__);
     exit(1);
   }
+  to_write = (size_t) chars_printed;
 
   if(line_num_failures == NULL)
   {