src: initialize some fields to suppress harmless valgrind errors

src/util/util.h

@@ -149,6 +149,7 @@ noinline const char *upx_getenv(const char *envvar) noexcept;
 
 void upx_memswap(void *a, void *b, size_t bytes) noexcept;
 
+noinline void upx_rand_init(void) noexcept;
 noinline int upx_rand(void) noexcept;
 
 typedef int(__acc_cdecl_qsort *upx_compare_func_t)(const void *, const void *);