memwipe: don't call the workhorse for 0 bytes

Some of them don't like it

contrib/epee/src/memwipe.c

@@ -50,7 +50,7 @@
 
 void *memwipe(void *ptr, size_t n)
 {
-  if (memset_s(ptr, n, 0, n))
+  if (n > 0 && memset_s(ptr, n, 0, n))
   {
 #ifdef NDEBUG
     fprintf(stderr, "Error: memset_s failed\n");
@@ -67,6 +67,7 @@ void *memwipe(void *ptr, size_t n)
 
 void *memwipe(void *ptr, size_t n)
 {
+  if (n > 0)
     explicit_bzero(ptr, n);
   SCARECROW
   return ptr;
@@ -105,6 +106,7 @@ static void memory_cleanse(void *ptr, size_t len)
 
 void *memwipe(void *ptr, size_t n)
 {
+  if (n > 0)
     memory_cleanse(ptr, n);
   SCARECROW
   return ptr;