box: fix assert with multikey hybrid index

Tarantool used to assume that offset_slot has an extension
iff field_map_get_offset is called with multikey_idx >= 0.
In fact, when some part of the index contains a multikey index
placeholder, tuple_compare_* routines pass a tuple_hint in
meaning of multikey index for each tuple_field_raw_by_part call,
even for regular key_part that doesn't have array index
placeholder (and, correspondingly, field_map extension).
Thus this assumption is invalid.

This patch uses the fact that field_map slots that have extension
store negative offset to distinguish multikey and normal usage
of the field_map_get_offset routine.

Closes #4234

src/box/field_map.h

@@ -152,8 +152,8 @@ field_map_get_offset(const uint32_t *field_map, int32_t offset_slot,
 		     int multikey_idx)
 {
 	uint32_t offset;
-	if (multikey_idx != MULTIKEY_NONE && field_map[offset_slot] > 0) {
-		assert((int32_t)field_map[offset_slot] < 0);
+	if (multikey_idx != MULTIKEY_NONE &&
+	    (int32_t) field_map[offset_slot] < 0) {
 		/**
 		 * The field_map extent has the following
 		 * structure: [size=N|slot1|slot2|..|slotN]

test/engine/multikey.result

@@ -753,3 +753,27 @@ i2:select()
 s:drop()
 ---
 ...
+--
+-- gh-4234: Assert when using indexes containing both multikey
+--          and regular key_parts.
+--
+s = box.schema.space.create('clients', {engine = engine})
+---
+...
+name_idx = s:create_index('name_idx', {parts = {{1, 'string'}}})
+---
+...
+phone_idx = s:create_index('phone_idx', {parts = {{'[2][*]', 'string'}, {3, 'string'}}, unique=false})
+---
+...
+s:insert({"Genadiy", {"911"}, 'b'})
+---
+- ['Genadiy', ['911'], 'b']
+...
+s:insert({"Jorge", {"911", "89457609234"}, 'a'})
+---
+- ['Jorge', ['911', '89457609234'], 'a']
+...
+s:drop()
+---
+...

test/engine/multikey.test.lua

@@ -194,3 +194,14 @@ s:replace{2, {{2, 3}}}
 i2 = s:create_index('sk', {parts = {{2, 'unsigned', path = '[1][*]'}}})
 i2:select()
 s:drop()
+
+--
+-- gh-4234: Assert when using indexes containing both multikey
+--          and regular key_parts.
+--
+s = box.schema.space.create('clients', {engine = engine})
+name_idx = s:create_index('name_idx', {parts = {{1, 'string'}}})
+phone_idx = s:create_index('phone_idx', {parts = {{'[2][*]', 'string'}, {3, 'string'}}, unique=false})
+s:insert({"Genadiy", {"911"}, 'b'})
+s:insert({"Jorge", {"911", "89457609234"}, 'a'})
+s:drop()