flipdiff: avoid crash on non-commuting patches

Makefile.am

@@ -286,6 +286,7 @@ TESTS = tests/newline1/run-test \
 	tests/flip17/run-test \
 	tests/flip18/run-test \
 	tests/flip19/run-test \
+	tests/flip20/run-test \
 	tests/unline1/run-test \
 	tests/nul0/run-test \
 	tests/nul1/run-test \

src/interdiff.c

@@ -1661,6 +1661,15 @@ remove_line (struct lines_info *lines, const char *line,
 		}
 	}
 
+	if (!kill) {
+		/* The line we were asked to remove is not present (for
+		 * example when the two patches do not commute).  There is
+		 * nothing to do. */
+		if (debug)
+			printf ("Nothing to remove at %lu: %s", n, line);
+		return;
+	}
+
 	at = kill;
 	while (at) {
 		at->n--;

tests/flip20/run-test

@@ -0,0 +1,30 @@
+#!/bin/sh
+
+# This is a flipdiff(1) testcase.
+# Test: non-commuting patches where the second patch removes a line that
+# the first patch had added.  flipdiff used to crash on this input.
+
+
+. ${top_srcdir-.}/tests/common.sh
+
+cat << EOF > file.orig
+a
+b
+EOF
+
+cat << EOF > file
+a-m
+b-y
+EOF
+
+${DIFF} -u file.orig file > patch1
+
+mv -f file file.orig
+cat << EOF > file
+a
+b-y
+EOF
+
+${DIFF} -u file.orig file > patch2
+
+${FLIPDIFF} patch1 patch2 > patch-flipped || exit 1