MD5 Verification fails on HTTPUpdate after unsuccessful OTAUpdate

[CriPstian]

Basic Infos

• This issue complies with the issue POLICY doc.
• I have read the documentation at readthedocs and the issue is not addressed there.
• I have tested that the issue is present in current master branch (aka latest git).
• I have searched the issue tracker for a similar issue.
• If there is a stack dump, I have decoded it.
• I have filled out all fields below.

Platform

• Hardware: [ESP8266]
• Core Version: [a543262]
• Development Env: [CLion]
• Operating System: [Ubuntu|]

Settings in IDE

• Module: [Nodemcu]
• Flash Mode: [qio|dio|other]
• Flash Size: [4MB]
• lwip Variant: [?]
• Reset Method: [nodemcu]
• Flash Frequency: [40Mhz]
• CPU Frequency: [80Mhz]
• Upload Using: [OTA]
• Upload Speed: [9600]

Problem Description

MD5 Verification fails on HTTP Update (with no x-MD5 header response) after unsuccessful OTA Update because the _target_md5 string in Updater class is not restored to default value.

MCVE Sketch

No need for a sketch, This is a reproducible issue by following two basic steps:

1. Fail an OTAUpdate by cancelling the update in the middle of the process
2. Trigger an HTTPUpdate with a server that has no x-MD5 header set on the response.
   => The update will fail with MD5 Check fail

Debug Messages

MD5 Failed: expected:<_target_md5>, calculated:<md5>

[earlephilhower]

Fixed via #7534