Fixed small buffer in get_datetime()

The buffer being hardcoded to 19 (the size expected from the default time format),
strftime() would fail on any format returning a longer buffer.
Changed it from 19 to 64 to accomodate longer formats.

slstatus.c

@@ -143,7 +143,7 @@ char *
 get_datetime()
 {
     time_t tm;
-    size_t bufsize = 19;
+    size_t bufsize = 64;
     char *buf = malloc(bufsize);
 
     /* get time in format */