diff3: fix heap use-after-free; add minimal diff3 test coverage

Commit v3.3-42-g3b74a90, "FIXME: src/diff3: plug a leak" added an
invalid use of free, leading to use-after-free in nearly any invocation
of diff3.  Revert that commit.
* NEWS (Bug fixes): Mention it.
* tests/diff3: New file, to add minimal test coverage.
* tests/Makefile.am (TESTS): Add it.
Reported by Bastian Beischer in http://bugs.gnu.org/24210

NEWS

@@ -4,6 +4,9 @@ GNU diffutils NEWS                                    -*- outline -*-
 
 ** Bug fixes
 
+  diff3 no longer malfunctions due to use-after-free
+  [bug introduced in 3.4]
+
   diff --color no longer colorizes when TERM=dumb
 
 

src/diff3.c

@@ -1039,7 +1039,6 @@ process_diff (char const *filea,
 
   *block_list_end = NULL;
   *last_block = bptr;
-  free (diff_contents);
   return block_list;
 }
 

tests/Makefile.am

@@ -6,6 +6,7 @@ TESTS = \
   binary \
   brief-vs-stat-zero-kernel-lies \
   colliding-file-names \
+  diff3 \
   excess-slash \
   help-version	\
   function-line-vs-leading-space \

tests/diff3

@@ -0,0 +1,30 @@
+#!/bin/sh
+# This would malfunction in diff-3.4
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+
+echo a > a || framework_failure_
+echo b > b || framework_failure_
+echo c > c || framework_failure_
+cat <<'EOF' > exp || framework_failure_
+====
+1:1c
+  a
+2:1c
+  b
+3:1c
+  c
+EOF
+
+fail=0
+
+diff3 a b c > out 2> err || fail=1
+compare exp out || fail=1
+compare /dev/null err || fail=1
+
+# Repeat, but with all three files the same:
+diff3 a a a > out 2> err || fail=1
+compare /dev/null out || fail=1
+compare /dev/null err || fail=1
+
+Exit $fail