psu-ng: Do not add trailing \0 to VINI SN

The SerialNumber property in the asset decorator properties has the
correct length, but for some reason a null (\0) was added to the end of
the value we send out to the VINI SN property. Doing this results in an
incorrect VPD data length.

Tested:
    Built fully p10bmc image, flashed to Rainier 2S2U.
    Used busctl introspect to examine powersupply properties.
    Powered on system
    Used PHYP macro to verifyvpd

Change-Id: Id82014c656161bb5e59b2216d7da80c09c3c7f29
Signed-off-by: Brandon Wyman <bjwyman@gmail.com>
diff --git a/phosphor-power-supply/power_supply.cpp b/phosphor-power-supply/power_supply.cpp
index 7bc1cda..a7e3579 100644
--- a/phosphor-power-supply/power_supply.cpp
+++ b/phosphor-power-supply/power_supply.cpp
@@ -885,7 +885,7 @@
                                 std::vector<uint8_t>(pn.begin(), pn.end()));
         ipzvpdVINIProps.emplace("FN",
                                 std::vector<uint8_t>(fn.begin(), fn.end()));
-        std::string header_sn = header + sn + '\0';
+        std::string header_sn = header + sn;
         ipzvpdVINIProps.emplace(
             "SN", std::vector<uint8_t>(header_sn.begin(), header_sn.end()));
         std::string description = "IBM PS";