utf8_to_uv_msgs: Make warning message consistent

This inserts a colon in the message to make it consistent with all the
other similar ones.

It is safe to do this, as this code is executed only as a fallback under
non-debugging builds.  The condition that gets to this warning is
asserted against in debugging builds.

Author: khwilliamson

utf8.c

@@ -2239,8 +2239,7 @@ Perl_utf8_to_uv_msgs_helper_(const U8 * const s0,
                  * caller.  If you have nothing to decode, skip calling this
                  * function */
 
-                assert(0);
-                message = Perl_form(aTHX_ "%s (empty string)", malformed_text);
+                message = Perl_form(aTHX_ "%s: (empty string)", malformed_text);
                 break;
 
               case UTF8_GOT_CONTINUATION: