strxcpyx: do not access dest as an array

dest is a pointer to a string, not an array. Accessing
the "first element" just happens to work, but let's
be more careful.

src/basic/strxcpyx.c

@@ -41,7 +41,7 @@ size_t strnpcpy_full(char **dest, size_t size, const char *src, size_t len, bool
         if (ret_truncated)
                 *ret_truncated = truncated;
 
-        *dest[0] = '\0';
+        (*dest)[0] = '\0';
         return size;
 }