SymbolizedFrame::name is already null-terminated

SymbolizedFrame::name is already null-terminated

Summary:
... as it comes from Elf::getSymbolName, which returns null-terminated C
strings. So there's no need to copy it into a fixed-size buffer (and have a
buffer overflow, ouch).

Test Plan: folly/experimental/symbolizer_test, see what else "arc unit" gets us

Reviewed By: tconerly@fb.com

FB internal diff: D1286348