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, 5 insertions, 0 deletions

diff --git a/exec.c b/exec.c
index c2163e3..35bf061 100644
--- a/exec.c
+++ b/exec.c
@@ -160,6 +160,11 @@ struct bfs_exec *parse_bfs_exec(char **argv, enum bfs_exec_flags flags, const st
 	execbuf->tmpl_argv = argv + 1;
 	execbuf->tmpl_argc = i - 1;
 
+	if (execbuf->tmpl_argc == 0) {
+		cfprintf(cerr, "%{er}error: %s: Missing command.%{rs}\n", argv[0]);
+		goto fail;
+	}
+
 	execbuf->argv_cap = execbuf->tmpl_argc + 1;
 	execbuf->argv = malloc(execbuf->argv_cap*sizeof(*execbuf->argv));
 	if (!execbuf->argv) {