src: fix stack-buffer overflow for long exception lines
Long exception lines result in a stack buffer overflow or assertion abort because the assumption that snprintf not counts discarded chars is wrong and (off) >= sizeof(arrow).
==6098==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffe1a973220 at pc 0x000001a0b09a bp 0x7ffe1a972db0 sp 0x7ffe1a972da8
WRITE of size 1 at 0x7ffe1a973220 thread T0
#0 0x1a0b099 in node::AppendExceptionLine(node::Environment*, v8::Local<v8::Value>, v8::Local<v8::Message>) /home/skomski/Code/io.js/out/../src/node.cc:1409:16
#1 0x1a3e14a in node::ContextifyScript::New(v8::FunctionCallbackInfo<v8::Value> const&) /home/skomski/Code/io.js/out/../src/node_contextify.cc:492:9
Address 0x7ffe1a973220 is located in stack of thread T0 at offset 1120 in frame
#0 0x1a0a9af in node::AppendExceptionLine(node::Environment*, v8::Local<v8::Value>, v8::Local<v8::Message>) /home/skomski/Code/io.js/out/../src/node.cc:1334
This frame has 4 object(s):
[32, 56) 'scope'
[96, 1120) 'arrow' <== Memory access at offset 1120 overflows this variable
[1248, 2288) 'filename'
[2416, 3456) 'sourceline'