fix empty stack in deadlock error message #12411

Job	Run time
coverage report	7m 51s
bootstrap build	2m 55s
style checks	5m 5s
test (x86_64-unknown-linux-gnu)	18m 28s
test (aarch64-unknown-linux-gnu)	18m 43s
test (i686-pc-windows-msvc)	23m 52s
test (armv7-unknown-linux-gnueabihf)	16m 45s
test (i686-unknown-linux-gnu)	19m 21s
test (aarch64-pc-windows-msvc)	20m 46s
automatic pull from rustc	0s
test (aarch64-apple-darwin)	9m 9s
conclusion	4s
cronjob failure notification	0s
	2h 22m 59s

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