utf8n_to_uvchr_msgs: Change formal parameter

This makes the beginning of the string to examine totally const, and
then initializes the pointer to the position in the string being parsed
to this constant s0.  Making it totally const enforces that the function
doesn't try to change it; and this makes the next commit trivial.

Author: khwilliamson

embed.fnc

@@ -3639,7 +3639,7 @@ AMTdp	|UV	|utf8n_to_uvchr_error					\
 				|const U32 flags			\
 				|NULLOK U32 *errors
 ATdip	|UV	|utf8n_to_uvchr_msgs					\
-				|NN const U8 *s 			\
+				|NN const U8 * const s0 		\
 				|STRLEN curlen				\
 				|NULLOK STRLEN *retlen			\
 				|const U32 flags			\

inline.h

@@ -2976,7 +2976,7 @@ Perl_is_utf8_fixed_width_buf_loclen_flags(const U8 * const s,
 }
 
 PERL_STATIC_INLINE UV
-Perl_utf8n_to_uvchr_msgs(const U8 *s,
+Perl_utf8n_to_uvchr_msgs(const U8 * const s0,
                          STRLEN curlen,
                          STRLEN *retlen,
                          const U32 flags,
@@ -2999,7 +2999,6 @@ Perl_utf8n_to_uvchr_msgs(const U8 *s,
         *msgs = NULL;
     }
 
-    const U8 * s0;
 
     /* No calls from core pass in an empty string; non-core need a check */
 #ifdef PERL_CORE
@@ -3012,16 +3011,16 @@ Perl_utf8n_to_uvchr_msgs(const U8 *s,
         /* UTF-8 invariants are returned unchanged.  The code below is quite
          * capable of handling this, but this shortcuts this very common case
          * */
-        if (UTF8_IS_INVARIANT(*s)) {
+        if (UTF8_IS_INVARIANT(*s0)) {
             if (retlen) {
                 *retlen = 1;
             }
 
-            return *s;
+            return *s0;
         }
 
+        const U8 * s = s0;
         const U8 * send = s + curlen;
-        s0 = s;
 
         /* This dfa is fast.  If it accepts the input, it was for a
          * well-formed, non-problematic code point, which can be returned