db.pg: fix invalid memory access in res_to_rows (#20248)

vlib/db/pg/pg.c.v

@@ -190,8 +190,7 @@ fn res_to_rows(res voidptr) []Row {
 				row.vals << none
 			} else {
 				val := C.PQgetvalue(res, i, j)
-				sval := unsafe { val.vstring() }
-				row.vals << sval
+				row.vals << unsafe { cstring_to_vstring(val) }
 			}
 		}
 		rows << row

vlib/db/pg/pg_test.v

@@ -0,0 +1,24 @@
+module main
+
+import db.pg
+
+const query = 'SELECT ischema.table_schema, c.relname, a.attname, t.typname, t.typalign, t.typlen
+  FROM pg_class c
+  JOIN information_schema.tables ischema on ischema.table_name = c.relname
+  JOIN pg_attribute a ON (a.attrelid = c.oid)
+  JOIN pg_type t ON (t.oid = a.atttypid)
+WHERE
+  a.attnum >= 0'
+
+fn test_large_exec() {
+	db := pg.connect(pg.Config{ user: 'postgres', password: 'secret', dbname: 'postgres' })!
+	defer {
+		db.close()
+	}
+
+	rows := db.exec(query)!
+	for row in rows {
+		// We just need to access the memory to ensure it's properly allocated
+		row.str()
+	}
+}