Fix logic in printing

`cbopaque` can now be overriden without overriding `write_cb` in
the first place.  (Otherwise there would be no need to have the
`cbopaque` parameter in `malloc_message`.)

doc/jemalloc.xml.in

@@ -424,7 +424,7 @@ for (i = 0; i < nbins; i++) {
       called repeatedly.  General information that never changes during
       execution can be omitted by specifying <quote>g</quote> as a character
       within the <parameter>opts</parameter> string.  Note that
-      <function>malloc_message()</function> uses the
+      <function>malloc_stats_print()</function> uses the
       <function>mallctl*()</function> functions internally, so inconsistent
       statistics can be reported if multiple threads use these functions
       simultaneously.  If <option>--enable-stats</option> is specified during

include/jemalloc/internal/malloc_io.h

@@ -54,7 +54,7 @@ size_t malloc_vsnprintf(char *str, size_t size, const char *format,
 size_t malloc_snprintf(char *str, size_t size, const char *format, ...)
     JEMALLOC_FORMAT_PRINTF(3, 4);
 /*
- * The caller can set write_cb and cbopaque to null to choose to print with the
+ * The caller can set write_cb to null to choose to print with the
  * je_malloc_message hook.
  */
 void malloc_vcprintf(void (*write_cb)(void *, const char *), void *cbopaque,

src/malloc_io.c

@@ -632,7 +632,6 @@ malloc_vcprintf(void (*write_cb)(void *, const char *), void *cbopaque,
 		 */
 		write_cb = (je_malloc_message != NULL) ? je_malloc_message :
 		    wrtmessage;
-		cbopaque = NULL;
 	}
 
 	malloc_vsnprintf(buf, sizeof(buf), format, ap);