WString: c_str() returns null pointer after move

[mcspr]

target = std::move(source) does not reset source's buffer pointer back to the sso
#7553 fixes this behaviour

Added as a separate test, perhaps it could go into the existing SSO

edit: failing, as intended:
https://github.com/esp8266/Arduino/pull/7611/checks?check_run_id=1166504473#step:4:2364

-------------------------------------------------------------------------------
core/test_string.cpp:22
...............................................................................

core/test_string.cpp:30: FAILED:
  REQUIRE( origin.c_str() != nullptr )
with expansion:
  {null string} != nullptr

[earlephilhower]

Can you add a patch that fixes this to the PR? I guess it was only discovered when the noexcept was added, allowing GCC to actually use the move method?

[mcspr]

Can you add a patch that fixes this to the PR?

Done as 7ce820a
Although, I also could rebase with --author=... rewrite of the git history and co-authred-by: $myself instead, since I repeat the existing patch almost 1-to-1

I guess it was only discovered when the noexcept was added, allowing GCC to actually use the move method?

Exactly. Which I noticed when running tests for the library assigning the existing object throughout the loop and moving it elsewhere periodically

[earlephilhower]

Good catch, thanks!