utils: Handle NULL consistently if memwipe() is implemented via explicit_bzero()

Our own implementation ignores NULL values, however, explicit_bzero()
can't handle that, as indicated by the `__nonnull ((1))` attribute in the
function's signature in string.h, and causes a segmentation fault.  This
was noticed in one of the unit tests for NewHope.  Since we usually use
memwipe() via chunk_clear(), which already ignores NULL pointers, this
is not that much of an issue in practice.

Fixes: 149d1bbb05 ("memory: Use explicit_bzero() as memwipe() if available")

src/libstrongswan/utils/utils/memory.h

@@ -87,7 +87,13 @@ static inline void *memset_noop(void *s, int c, size_t n)
 void memxor(uint8_t dest[], const uint8_t src[], size_t n);
 
 #ifdef HAVE_EXPLICIT_BZERO
-#define memwipe(ptr, n) explicit_bzero(ptr, n)
+static inline void memwipe(void *ptr, size_t n)
+{
+	if (ptr)
+	{
+		explicit_bzero(ptr, n);
+	}
+}
 #else /* HAVE_EXPLICIT_BZERO */
 /**
  * Safely overwrite n bytes of memory at ptr with zero, non-inlining variant.