SEGV in wabt::Module::operator=

GIT revision: ed1ce97e
Built with USE_ASAN=ON
Run: ./build/wat2wasm test.wat
Test:

(module quote "\7c")

Backtrace:

AddressSanitizer:DEADLYSIGNAL
=================================================================
==1100246==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x7f5a434be881 bp 0x7ffcc43aba80 sp 0x7ffcc43ab238 T0)
==1100246==The signal is caused by a READ memory access.
==1100246==Hint: address points to the zero page.
    #0 0x7f5a434be881 in memcpy string/../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:222
    #1 0x55ab4a3b93c4 in __asan_memcpy (wabt/bin/wat2wasm+0x1823c4) (BuildId: a5f593cb6819aa12ce15795bdae38dd1e63c168d)
    #2 0x55ab4a5467b5 in wabt::Module::operator=(wabt::Module&&) (wabt/bin/wat2wasm+0x30f7b5) (BuildId: a5f593cb6819aa12ce15795bdae38dd1e63c168d)
    #3 0x55ab4a508c3c in wabt::WastParser::ParseModuleCommand(wabt::Script*, std::unique_ptr<wabt::Command, std::default_delete<wabt::Command>>*) (wabt/bin/wat2wasm+0x2d1c3c) (BuildId: a5f593cb6819aa12ce15795bdae38dd1e63c168d)
    #4 0x55ab4a506fdc in wabt::WastParser::ParseModule(std::unique_ptr<wabt::Module, std::default_delete<wabt::Module>>*) (wabt/bin/wat2wasm+0x2cffdc) (BuildId: a5f593cb6819aa12ce15795bdae38dd1e63c168d)
    #5 0x55ab4a53d611 in wabt::ParseWatModule(wabt::WastLexer*, std::unique_ptr<wabt::Module, std::default_delete<wabt::Module>>*, std::vector<wabt::Error, std::allocator<wabt::Error>>*, wabt::WastParseOptions*) (wabt/bin/wat2wasm+0x306611) (BuildId: a5f593cb6819aa12ce15795bdae38dd1e63c168d)
    #6 0x55ab4a3fa8f9 in ProgramMain(int, char**) (wabt/bin/wat2wasm+0x1c38f9) (BuildId: a5f593cb6819aa12ce15795bdae38dd1e63c168d)
    #7 0x55ab4a3fc2c1 in main (wabt/bin/wat2wasm+0x1c52c1) (BuildId: a5f593cb6819aa12ce15795bdae38dd1e63c168d)
    #8 0x7f5a43423d8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #9 0x7f5a43423e3f in __libc_start_main csu/../csu/libc-start.c:392:3
    #10 0x55ab4a31f194 in _start (wabt/bin/wat2wasm+0xe8194) (BuildId: a5f593cb6819aa12ce15795bdae38dd1e63c168d)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV string/../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:222 in memcpy
==1100246==ABORTING

The issue was found by libFuzzer with a Grammarinator-based custom mutator.

Run ./build/wat2wasm test.wat with the shown module input under USE_ASAN=ON, then inspect wabt::Module::operator= and the WastParser::ParseModuleCommand path from the backtrace. The fix is done when this input no longer triggers the SEGV and equivalent regression coverage passes under ASAN.