Fix return type of string_bytes

The function actually returns `ptrdiff_t` (and the callers already
expected that type).

Author: mgeisler

elisp.c

@@ -79,7 +79,7 @@ emacs_value symbol_value(emacs_env *env, emacs_value symbol) {
   return env->funcall(env, Fsymbol_value, 1, (emacs_value[]){symbol});
 }
 
-int string_bytes(emacs_env *env, emacs_value string) {
+ptrdiff_t string_bytes(emacs_env *env, emacs_value string) {
   ptrdiff_t size = 0;
   env->copy_string_contents(env, string, NULL, &size);
   return size;

elisp.h

@@ -65,7 +65,7 @@ extern emacs_value Fvterm_set_selection;
 void bind_function(emacs_env *env, const char *name, emacs_value Sfun);
 void provide(emacs_env *env, const char *feature);
 emacs_value symbol_value(emacs_env *env, emacs_value symbol);
-int string_bytes(emacs_env *env, emacs_value string);
+ptrdiff_t string_bytes(emacs_env *env, emacs_value string);
 emacs_value length(emacs_env *env, emacs_value string);
 emacs_value list(emacs_env *env, emacs_value elements[], ptrdiff_t len);
 emacs_value nth(emacs_env *env, int idx, emacs_value list);