sdbm.c: fix off-by-one access to global ".dir"

Detected by clang -faddress-sanitizer.

The bug came in 081f72a, where
we started calculating lengths with sizeof on string constants
instead of using strlen.  Since string constants include the null
byte, sizeof(".dir"), for example, is 5, but we've been copying 6
bytes.

This patch resolves [perl #111586] and includes revisions by the
committer.

ext/SDBM_File/sdbm/sdbm.c

@@ -78,8 +78,8 @@ sdbm_open(register char *file, register int flags, register int mode)
 	register char *dirname;
 	register char *pagname;
 	size_t filelen;
-	const size_t dirfext_len = sizeof(DIRFEXT "");
-	const size_t pagfext_len = sizeof(PAGFEXT "");
+	const size_t dirfext_size = sizeof(DIRFEXT "");
+	const size_t pagfext_size = sizeof(PAGFEXT "");
 
 	if (file == NULL || !*file)
 		return errno = EINVAL, (DBM *) NULL;
@@ -88,17 +88,17 @@ sdbm_open(register char *file, register int flags, register int mode)
  */
 	filelen = strlen(file);
 
-	if ((dirname = (char *) malloc(filelen + dirfext_len + 1
-				       + filelen + pagfext_len + 1)) == NULL)
+	if ((dirname = (char *) malloc(filelen + dirfext_size
+				       + filelen + pagfext_size)) == NULL)
 		return errno = ENOMEM, (DBM *) NULL;
 /*
  * build the file names
  */
 	memcpy(dirname, file, filelen);
-	memcpy(dirname + filelen, DIRFEXT, dirfext_len + 1);
-	pagname = dirname + filelen + dirfext_len + 1;
+	memcpy(dirname + filelen, DIRFEXT, dirfext_size);
+	pagname = dirname + filelen + dirfext_size;
 	memcpy(pagname, file, filelen);
-	memcpy(pagname + filelen, PAGFEXT, pagfext_len + 1);
+	memcpy(pagname + filelen, PAGFEXT, pagfext_size);
 
 	db = sdbm_prep(dirname, pagname, flags, mode);
 	free((char *) dirname);