[ruby-core:104957] [Ruby master Bug#18076] ext/digest/md5/md5.c: `-Wnull-pointer-subtraction` warning by Clang 13

Issue #18076 has been updated by xtkoba (Tee KOBAYASHI).


Proposed patch:

```diff
--- a/ext/digest/md5/md5.c
+++ b/ext/digest/md5/md5.c
@@ -225,7 +225,7 @@
     uint32_t xbuf[16];
     const uint32_t *X;
 
-    if (!((data - (const uint8_t *)0) & 3)) {
+    if (!(((uintptr_t)data) & 3)) {
 	/* data are properly aligned */
 	X = (const uint32_t *)data;
     } else {
```

----------------------------------------
Bug #18076: ext/digest/md5/md5.c: `-Wnull-pointer-subtraction` warning by Clang 13
https://bugs.ruby-lang.org/issues/18076#change-93319

* Author: xtkoba (Tee KOBAYASHI)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.1.0dev (2021-08-16T00:07:09Z master 4cc44bd819) [x86_64-linux]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
```
compiling ../../../../ext/digest/md5/md5.c
../../../../ext/digest/md5/md5.c:228:17: warning: performing pointer subtraction with a null pointer has undefined behavior [-Wnull-pointer-subtraction]
    if (!((data - (const uint8_t *)0) & 3)) {
                ^ ~~~~~~~~~~~~~~~~~~
1 warning generated.
```



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