fixed some simple warnings

Author: tarun

Sources/Fuzzilli/Base/ProgramBuilder.swift

@@ -1158,7 +1158,7 @@ public class ProgramBuilder {
             return nil
         }
 
-        var weightedTypes = WeightedList<ILType>()
+        let weightedTypes = WeightedList<ILType>() // set this to let to avoid warning 
         for (t, w) in visibleTypes {
             weightedTypes.append(t, withWeight: w)
         }

Sources/Fuzzilli/Database/PostgreSQLStorage.swift

@@ -332,7 +332,7 @@ public class PostgreSQLStorage {
                 }
             } catch {
                 // Rollback on error
-                try? await connection.query("ROLLBACK", logger: self.logger)
+                _ = try? await connection.query("ROLLBACK", logger: self.logger)
                 throw error
             }
         }

Sources/Fuzzilli/Util/WeightedList.swift

@@ -41,7 +41,7 @@ public class WeightedList<Element>: Sequence {
     }
 
     public func filter(_ isIncluded: (Element) -> Bool) -> WeightedList<Element> {
-        var r = WeightedList()
+        let r = WeightedList() // set this to let to avoid warning
         for (e, w, _) in elements where isIncluded(e) {
             r.append(e, withWeight: w)
         }