Fix CVE-2017-8291:

https://www.cve.mitre.org/cgi-bin/cvename.cgi?name=2017-8291

This patch is adapted from these two Artifex Ghostscript commits by Leo
Famulari <leo@famulari.name>:

https://git.ghostscript.com/?p=ghostpdl.git;a=commitdiff;h=04b37bbce174eed24edec7ad5b920eb93db4d47d;hp=4f83478c88c2e05d6e8d79ca4557eb039354d2f3
https://git.ghostscript.com/?p=ghostpdl.git;a=commitdiff;h=4f83478c88c2e05d6e8d79ca4557eb039354d2f3;hp=5603e8fc3e59c435318877efe627967ee6baebb8

diff --git a/psi/zfrsd.c b/psi/zfrsd.c
index fb4bce9..2629afa 100644
--- a/psi/zfrsd.c
+++ b/psi/zfrsd.c
@@ -49,13 +49,20 @@ zrsdparams(i_ctx_t *i_ctx_p)
     ref *pFilter;
     ref *pDecodeParms;
     int Intent = 0;
-    bool AsyncRead;
+    bool AsyncRead = false;
     ref empty_array, filter1_array, parms1_array;
     uint i;
-    int code;
+    int code = 0;
+
+    if (ref_stack_count(&o_stack) < 1)
+        return_error(e_stackunderflow);
+    if (!r_has_type(op, t_dictionary) && !r_has_type(op, t_null)) {
+        return_error(e_typecheck);
+    }
 
     make_empty_array(&empty_array, a_readonly);
-    if (dict_find_string(op, "Filter", &pFilter) > 0) {
+    if (r_has_type(op, t_dictionary)
+        && dict_find_string(op, "Filter", &pFilter) > 0) {
         if (!r_is_array(pFilter)) {
             if (!r_has_type(pFilter, t_name))
                 return_error(e_typecheck);
@@ -94,12 +101,13 @@ zrsdparams(i_ctx_t *i_ctx_p)
                 return_error(e_typecheck);
         }
     }
-    code = dict_int_param(op, "Intent", 0, 3, 0, &Intent);
+    if (r_has_type(op, t_dictionary))
+        code = dict_int_param(op, "Intent", 0, 3, 0, &Intent);
     if (code < 0 && code != e_rangecheck) /* out-of-range int is ok, use 0 */
         return code;
-    if ((code = dict_bool_param(op, "AsyncRead", false, &AsyncRead)) < 0
-        )
-        return code;
+    if (r_has_type(op, t_dictionary))
+        if ((code = dict_bool_param(op, "AsyncRead", false, &AsyncRead)) < 0)
+            return code;
     push(1);
     op[-1] = *pFilter;
     if (pDecodeParms)
diff --git a/psi/zmisc3.c b/psi/zmisc3.c
index 54b3042..0d357f1 100644
--- a/psi/zmisc3.c
+++ b/psi/zmisc3.c
@@ -56,6 +56,12 @@ zeqproc(i_ctx_t *i_ctx_p)
     ref2_t stack[MAX_DEPTH + 1];
     ref2_t *top = stack;
 
+    if (ref_stack_count(&o_stack) < 2)
+        return_error(e_stackunderflow);
+    if (!r_is_array(op - 1) || !r_is_array(op)) {
+        return_error(e_typecheck);
+    }
+
     make_array(&stack[0].proc1, 0, 1, op - 1);
     make_array(&stack[0].proc2, 0, 1, op);
     for (;;) {
