Fix REALLOC_ZERO_BYTES_FREES comment

[Paul Eggert <eggert@cs.ucla.edu>]

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While looking into problems with glibc's documentation of 
malloc/free/etc. I noticed that the comment for REALLOC_ZERO_BYTES_FREES 
was out of date: its reference to "the C standard" refers to ISO C11, 
but that compatibility problem has been fixed in C17. I installed the 
attached commentary fix as obvious.

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From dff9e592b8f74e2e7be015cbee1c0fad3ef96d37 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Sun, 11 Apr 2021 14:39:20 -0700
Subject: [PATCH] Fix REALLOC_ZERO_BYTES_FREES comment to match C17

* malloc/malloc.c (REALLOC_ZERO_BYTES_FREES):
Update comment to match current C standard.
---
 malloc/malloc.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/malloc/malloc.c b/malloc/malloc.c
index 6640385282..0cd3ba78ca 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -346,10 +346,13 @@ __malloc_assert (const char *assertion, const char *file, unsigned int line,
 #define REVEAL_PTR(ptr)  PROTECT_PTR (&ptr, ptr)
 
 /*
-  REALLOC_ZERO_BYTES_FREES should be set if a call to
-  realloc with zero bytes should be the same as a call to free.
-  This is required by the C standard. Otherwise, since this malloc
-  returns a unique pointer for malloc(0), so does realloc(p, 0).
+  REALLOC_ZERO_BYTES_FREES controls the behavior of realloc (p, 0)
+  when p is nonnull.  If nonzero, realloc (p, 0) should free p and
+  return NULL.  Otherwise, realloc (p, 0) should do the equivalent
+  of freeing p and returning what malloc (0) would return.
+
+  ISO C17 says the behavior is implementation-defined here; glibc
+  follows historical practice and defines it to be nonzero.
 */
 
 #ifndef REALLOC_ZERO_BYTES_FREES
-- 
2.27.0