check for NULL in get_fmt_mapping (fixes #1105)

When there is no format to be found after a %, get_fmt_mapping returns
NULL. It then gets called against strlen, which is a typical SEGV.
Check for NULL aginst mapping, which signals a null format being
specified.

src/core/pp.c

@@ -811,6 +811,7 @@ static const char *scanformat(
     while (p2 <= p) {
         if (strchr(FMT_REPLACE_INTTYPES, *p2) != NULL) {
             const char *mapping = get_fmt_mapping(*p2++);
+            if (!mapping) janet_panic("invalid format (found null)");
             size_t len = strlen(mapping);
             strcpy(form, mapping);
             form += len;