MDEV-27058 fixup: GCC 11 -march=i686 -Warray-bounds

page_zip_des_t::clear(): Avoid a bogus GCC warning with
some pointer arithmetics. Yes, storing the unrelated member "fix"
in this object is ugly, but it avoids memory alignment overhead
on 64-bit architectures.

Author: dr-m

storage/innobase/include/page0types.h

@@ -113,9 +113,10 @@ struct page_zip_des_t
 #endif /* UNIV_DEBUG */
 
 	void clear() {
-		memset((void*) this, 0, sizeof(data) + sizeof(uint32_t));
-		ut_d(m_start = 0);
-		ut_d(m_external = false);
+		/* Clear everything except the member "fix". */
+		memset((void*) this, 0,
+		       reinterpret_cast<char*>(&fix)
+		       - reinterpret_cast<char*>(this));
 	}
 
 private: