variant_under_utf8_count: mv ARGS_ASSERT

Better to have this first in the function

Author: khwilliamson

inline.h

@@ -2062,11 +2062,11 @@ C<L<perlapi/is_utf8_invariant_string_loc>>,
 PERL_STATIC_INLINE Size_t
 S_variant_under_utf8_count(const U8* const s, const U8* const e)
 {
+    PERL_ARGS_ASSERT_VARIANT_UNDER_UTF8_COUNT;
+
     const U8* x = s;
     Size_t count = 0;
 
-    PERL_ARGS_ASSERT_VARIANT_UNDER_UTF8_COUNT;
-
 #  ifndef EBCDIC
 
     /* Test if the string is long enough to use word-at-a-time.  (Logic is the