poison: added valgrind support

If the header valgrind/memcheck.h is available, add
VALGRIND_MAKE_MEM_NOACCESS() and VALGRIND_MAKE_MEM_UNDEFINED()
support, which enables nice warnings in the valgrind memory checker.

src/poison.h

@@ -20,6 +20,12 @@
 #define MPD_POISON_H
 
 #ifndef NDEBUG
+#include "config.h"
+
+#ifdef HAVE_VALGRIND_MEMCHECK_H
+#include <valgrind/memcheck.h>
+#endif
+
 #include <string.h>
 #endif
 
@@ -37,6 +43,10 @@ poison_noaccess(void *p, size_t length)
 	(void)length;
 #else
 	memset(p, 0x01, length);
+
+#ifdef HAVE_VALGRIND_MEMCHECK_H
+	VALGRIND_MAKE_MEM_NOACCESS(p, length);
+#endif
 #endif
 }
 
@@ -54,6 +64,10 @@ poison_undefined(void *p, size_t length)
 	(void)length;
 #else
 	memset(p, 0x02, length);
+
+#ifdef HAVE_VALGRIND_MEMCHECK_H
+	VALGRIND_MAKE_MEM_UNDEFINED(p, length);
+#endif
 #endif
 }