MONGOCRYPT-625 address Coverity issues (#838)

* initialize `op`

To silence the Coverity issue with CID 156409. Though this appears to be OK as-is.

* zero-init `bson_iter_t`

To silence Coverity issue with CID 156391.

* initialize `result_compactionTokens`

To silience Coverity issue with CID 156424.

src/mc-fle2-rfds.c

@@ -217,7 +217,7 @@ bool mc_FLE2RangeFindDriverSpec_parse(mc_FLE2RangeFindDriverSpec_t *spec,
     // {$and: [{$gt: ["$age", 5]}, {$lt:["$age", 50]}]}
     // Or `in` may be a Match Expression with this form:
     // {$and: [{age: {$gt: 5}}, {age: {$lt: 50}} ]}
-    bson_iter_t and, array;
+    bson_iter_t and = {0}, array = {0};
     bool ok = false;
 
     if (!parse_and(in, &and, status)) {
@@ -264,7 +264,7 @@ bool mc_FLE2RangeFindDriverSpec_parse(mc_FLE2RangeFindDriverSpec_t *spec,
             }
         }
 
-        operator_value_t op;
+        operator_value_t op = {0};
         switch (arg_type) {
         case AGGREGATE_EXPRESSION:
             if (!parse_aggregate_expression(in, &doc, &op, status)) {

src/mongocrypt-ctx-encrypt.c

@@ -1434,7 +1434,7 @@ static bool _fle2_append_compactionTokens(mongocrypt_t *crypt,
                                           const char *command_name,
                                           bson_t *out,
                                           mongocrypt_status_t *status) {
-    bson_t result_compactionTokens;
+    bson_t result_compactionTokens = BSON_INITIALIZER;
     bool ret = false;
 
     BSON_ASSERT_PARAM(crypt);