Fix panic in backtrace symbolication on win7 #807

Job	Run time
Windows AArch64	58s
Test (ubuntu-20.04, stable)	1m 52s
Test (ubuntu-20.04, beta)	2m 4s
Build Targets (wasm32-unknown-unknown)	17s
Docker (aarch64-unknown-linux-gnu)	52s
Test (ubuntu-20.04, nightly)	2m 4s
Build Targets (wasm32-wasi)	17s
Docker (arm-unknown-linux-gnueabihf)	41s
Test (macos-latest, stable)	4m 7s
Docker (armv7-unknown-linux-gnueabihf)	49s
Test (macos-latest, nightly)	4m 21s
Docker (i586-unknown-linux-gnu)	39s
Build Targets (x86_64-unknown-fuchsia)	17s
Test (windows-latest, stable-x86_64-msvc)	1m 45s
iOS (aarch64-apple-ios, iphoneos)	38s
Docker (i686-unknown-linux-gnu)	56s
Test (windows-latest, stable-i686-msvc)	2m 22s
iOS (x86_64-apple-ios, iphonesimulator)	50s
Build Targets (x86_64-fortanix-unknown-sgx)	16s
Docker (powerpc64-unknown-linux-gnu)	52s
Build Targets (x86_64-unknown-illumos)	21s
Test (windows-latest, stable-x86_64-gnu)	2m 30s
Docker (s390x-unknown-linux-gnu)	47s
Docker (x86_64-pc-windows-gnu)	52s
Miri	2m 16s
MSRV	3m 52s
Docker (x86_64-unknown-linux-gnu)	44s
Rustfmt	18s
Docker (x86_64-unknown-linux-musl)	37s
Docker (arm-linux-androideabi)	1m 24s
Docker (armv7-linux-androideabi)	1m 12s
Docker (aarch64-linux-android)	1m 20s
Docker (i686-linux-android)	1m 19s
Docker (x86_64-linux-android)	1m 30s
	45m 59s

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