fix "git apply --index ..." not to deref NULL

I noticed this when "git am CORRUPTED" unexpectedly failed with an
odd diagnostic, and even removed one of the files it was supposed
to have patched.

Reproduce with any valid old/new patch from which you have removed
the "+++ b/FILE" line.  You'll see a diagnostic like this

    fatal: unable to write file '(null)' mode 100644: Bad address

and you'll find that FILE has been removed.

The above is on glibc-based systems.  On other systems, rather than
getting "null", you may provoke a segfault as git tries to
dereference the NULL file name.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/apply.c

@@ -1405,6 +1405,9 @@ static int find_header(char *line, unsigned long size, int *hdrsize, struct patc
 					    "%d leading pathname components (line %d)" , p_value, linenr);
 				patch->old_name = patch->new_name = patch->def_name;
 			}
+			if (!patch->is_delete && !patch->new_name)
+				die("git diff header lacks filename information "
+				    "(line %d)", linenr);
 			patch->is_toplevel_relative = 1;
 			*hdrsize = git_hdr_len;
 			return offset;

t/t4254-am-corrupt.sh

@@ -0,0 +1,43 @@
+#!/bin/sh
+
+test_description='git am with corrupt input'
+. ./test-lib.sh
+
+# Note the missing "+++" line:
+cat > bad-patch.diff <<'EOF'
+From: A U Thor <au.thor@example.com>
+diff --git a/f b/f
+index 7898192..6178079 100644
+--- a/f
+@@ -1 +1 @@
+-a
++b
+EOF
+
+test_expect_success setup '
+	test $? = 0 &&
+	echo a > f &&
+	git add f &&
+	test_tick &&
+	git commit -m initial
+'
+
+# This used to fail before, too, but with a different diagnostic.
+#   fatal: unable to write file '(null)' mode 100644: Bad address
+# Also, it had the unwanted side-effect of deleting f.
+test_expect_success 'try to apply corrupted patch' '
+	git am bad-patch.diff 2> actual
+	test $? = 1
+'
+
+cat > expected <<EOF
+fatal: git diff header lacks filename information (line 4)
+EOF
+
+test_expect_success 'compare diagnostic; ensure file is still here' '
+	test $? = 0 &&
+	test -f f &&
+	test_cmp expected actual
+'
+
+test_done