Fortify hook routine

Add checks for syntax errors and missing SATURNIN_SUBCOMMAND.  In both
cases, ignore the hook.

src/bin/saturnin.skel

 }
 
 run_hook() {
-    inigrep -j -p hook.$SATURNIN_SUBCOMMAND.$1 \
-      | debug_pipe hook_$1 \
-      | bash
+    #
+    # Run custom hook
+    #
+    local hname="$1"
+    local hook_code
+    test -n "$SATURNIN_SUBCOMMAND" || {
+        warn "unknown subcommand, ignoring hook: $hname"
+        return 0
+    }
+    hook_code="$(inigrep -j -p hook.$SATURNIN_SUBCOMMAND.$hname)"
+    debug -v SATURNIN_SUBCOMMAND hook_code hname
+    bash -n <<<"$hook_code" || {
+        warn "syntax errors, ignoring hook: $hname"
+        return 0
+    }
+    eval "$hook_code"
 }
 
 subcommand() {