Update RNFetchBlobReq.java

fixed scoping problem

android/src/main/java/com/RNFetchBlob/RNFetchBlobReq.java

 //                    ignored.printStackTrace();
                 }
 
+                RNFetchBlobFileResp rnFetchBlobFileResp;
+                
                 try {
-                    RNFetchBlobFileResp rnFetchBlobFileResp = (RNFetchBlobFileResp) responseBody;
+                    rnFetchBlobFileResp = (RNFetchBlobFileResp) responseBody;
                 } catch (ClassCastException ex) {
                     // unexpected response type
                     if (responseBody != null) {