LibEDID: Return "Unknown" string if failed to determine the manufacturer

Before of this patch, It happened that the return string could be "@@@",
as a result of doing mathematical addition of ASCII '@' with bits when
decoding the packed manufacturer ID bytes from the EDID.
To avoid this, consider m_legacy_manufacturer_id to be invalid until we
successfully decode the packed bytes.

Userland/Libraries/LibEDID/EDID.h

@@ -457,6 +457,7 @@ private:
     String m_version;
 #endif
     char m_legacy_manufacturer_id[4] {};
+    bool m_legacy_manufacturer_id_valid { false };
 };
 
 }