sql: fix segfault in pragma table_info

We should first check that primary key is not NULL.

Closes #4745

(cherry picked from commit e9aa3784)

src/box/sql/pragma.c

@@ -238,10 +238,10 @@ sql_pragma_table_info(struct Parse *parse, const char *tbl_name)
 	struct Vdbe *v = sqlGetVdbe(parse);
 	struct field_def *field = space->def->fields;
 	for (uint32_t i = 0, k; i < space->def->field_count; ++i, ++field) {
-		if (!sql_space_column_is_in_pk(space, i)) {
-			k = 0;
-		} else if (pk == NULL) {
+		if (pk == NULL) {
 			k = 1;
+		} else if (!sql_space_column_is_in_pk(space, i)) {
+			k = 0;
 		} else {
 			struct key_def *kdef = pk->def->key_def;
 			k = key_def_find_by_fieldno(kdef, i) - kdef->parts + 1;

test/sql/gh-4745-table-info-assertion.result

+-- test-run result file version 2
+test_run = require('test_run').new()
+ | ---
+ | ...
+engine = test_run:get_cfg('engine')
+ | ---
+ | ...
+--
+-- Make sure that 'pragma table_info()' correctly handles tables
+-- without primary key.
+--
+T = box.schema.create_space('T', {         \
+    engine = engine,                       \
+    format = {{'i', 'integer'}}            \
+})
+ | ---
+ | ...
+_ = box.execute('pragma table_info(T)')
+ | ---
+ | ...
+T:drop()
+ | ---
+ | ...

test/sql/gh-4745-table-info-assertion.test.lua

+test_run = require('test_run').new()
+engine = test_run:get_cfg('engine')
+--
+-- Make sure that 'pragma table_info()' correctly handles tables
+-- without primary key.
+--
+T = box.schema.create_space('T', {         \
+    engine = engine,                       \
+    format = {{'i', 'integer'}}            \
+})
+_ = box.execute('pragma table_info(T)')
+T:drop()