Modify malloc_vsnprintf() validation code.

Modify malloc_vsnprintf() validation code to verify that output is
identical to vsnprintf() output, even if both outputs are truncated due
to buffer exhaustion.

include/jemalloc/internal/util.h

@@ -4,6 +4,12 @@
 /* Size of stack-allocated buffer passed to buferror(). */
 #define	BUFERROR_BUF		64
 
+/*
+ * Size of static buffer used by malloc_[v]{,c,t}printf().  This must be large
+ * enough for all possible uses within jemalloc.
+ */
+#define	MALLOC_PRINTF_BUFSIZE	4096
+
 /*
  * Define a custom assert() in order to reduce the chances of deadlock during
  * assertion failure.