exec: Reject -exec \; without a command

Prior to this, we'd fork and then segfault on every file as NULL was
passed to execvpe().  Found while looking through old FreeBSD find bugs:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=36521

bfs still supports the (dubious, possibly unintentional?) GNU find
extension to POSIX that allows

    $ bfs -exec {} \;

1 files changed, 15 insertions, 1 deletions

diff --git a/tests.sh b/tests.sh
index 4ced3c0..35f0224 100755
--- a/tests.sh
+++ b/tests.sh
@@ -439,6 +439,8 @@ gnu_tests=(
 
     test_empty
 
+
+    test_exec_nothing
     test_exec_substring
 
     test_execdir
@@ -498,6 +500,8 @@ gnu_tests=(
 
     test_nouser
 
+    test_ok_nothing
+
     test_okdir_plus_semicolon
 
     test_perm_000_slash
@@ -1122,7 +1126,12 @@ function test_size_big() {
 }
 
 function test_exec() {
-    bfs_diff basic -exec echo '{}' ';'
+    bfs_diff basic -exec echo '{}' \;
+}
+
+function test_exec_nothing() {
+    # Regression test: don't segfault on missing command
+    ! invoke_bfs basic -exec \; 2>/dev/null
 }
 
 function test_exec_plus() {
@@ -1362,6 +1371,11 @@ function test_not_prune() {
     bfs_diff basic \! \( -name foo -prune \)
 }
 
+function test_ok_nothing() {
+    # Regression test: don't segfault on missing command
+    ! invoke_bfs basic -ok \; 2>/dev/null
+}
+
 function test_ok_stdin() {
     # -ok should *not* close stdin
     # See https://savannah.gnu.org/bugs/?24561