geobuf.encode fails out of memory when encountering defective multi-MultiPolygons

Nobody has claimed this yet.

I'm having a weird problem where geobuf.encode is failing due to insufficient heap memory. At first, I assumed it was just an inherent problem with a huge file, but actually my input file is not that big. The JSON I'm converting is only 13MB minified, well below the 256MB string limit, and miles below various other limits of 1.6GB or 8GB I've heard of.

My code:

```
const geobuf = require('geobuf');
const json = require('./data/elb.json')
console.log(JSON.stringify(json).length)
const pbf = geobuf.encode(json, new (require('pbf'))());
const out = fs.createWriteStream('out.geobuf');
out.write(Buffer.from(pbf));
```

Running like this:

```
$ node --max-old-space-size=8192 --trace-gc-verbose justgeobuf.js
[47104:0x102801c00] Shrinking page 0x3110fdb80000: end 0x3110fdc00000 -> 0x3110fdbc9000
[47104:0x102801c00] Shrinking page 0x31107b480000: end 0x31107b500000 -> 0x31107b485000
[47104:0x102801c00] Fast promotion mode: false survival rate: 70%
[47104:0x102801c00] Fast promotion mode: false survival rate: 93%
[47104:0x102801c00] Fast promotion mode: false survival rate: 40%
[47104:0x102801c00] Fast promotion mode: false survival rate: 93%
[47104:0x102801c00] Fast promotion mode: false survival rate: 48%
[47104:0x102801c00] Fast promotion mode: false survival rate: 98%
[47104:0x102801c00] Fast promotion mode: false survival rate: 49%
13265241
[47104:0x102801c00] Fast promotion mode: false survival rate: 0%
[47104:0x102801c00] Fast promotion mode: false survival rate: 26%
[47104:0x102801c00] Fast promotion mode: false survival rate: 62%
[47104:0x102801c00] Fast promotion mode: false survival rate: 33%
[47104:0x102801c00] Fast promotion mode: false survival rate: 57%
[47104:0x102801c00] Fast promotion mode: false survival rate: 69%
[47104:0x102801c00] Fast promotion mode: false survival rate: 4%
[47104:0x102801c00] 1195 ms: Heap growing factor 4.0 based on mu=0.970, speed_ratio=0 (gc=0, mutator=465190)
[47104:0x102801c00] 1195 ms: Grow: old size: 381619 KB, new limit: 1542588 KB (4.0)

<--- Last few GCs --->

[47104:0x102801c00] 865 ms: Scavenge 134.3 (155.2) -> 127.9 (157.2) MB, 18.5 / 0.0 ms allocation failure
[47104:0x102801c00] 896 ms: Scavenge 136.7 (157.2) -> 132.1 (164.2) MB, 13.1 / 0.0 ms allocation failure
[47104:0x102801c00] 1195 ms: Mark-sweep 473.7 (503.2) -> 372.9 (441.1) MB, 3.2 / 0.0 ms (+ 1.8 ms in 5 steps since start of marking, biggest step 0.7 ms, walltime since start of marking 262 ms) finalize incremental marking via stack guard GC in old s

<--- JS stacktrace --->

==== JS stack trace =========================================

0: ExitFrame [pc: 0x1d902ef0427d]
Security context: 0x3110578a06a9
1: populateLine(aka populateLine) [/Users/stevebennett/odev/play/which-boundary/node_modules/geobuf/encode.js:~205] [pc=0x1d902eff99da](this=0x3110fdb822e1 ,coords=0x31108908d791 ,line=0x31105e756a69 ,closed=0x3110fdb82381 )
2: writeMultiPolygon(aka writeMultiPolygon) [/Users/stevebennett/odev/play/w...

FATAL ERROR: invalid array length Allocation failed - JavaScript heap out of memory
1: node::Abort() [/usr/local/bin/node]
2: node::FatalTryCatch::~FatalTryCatch() [/usr/local/bin/node]
3: v8::internal::V8::FatalProcessOutOfMemory(char const*, bool) [/usr/local/bin/node]
4: v8::internal::Heap::AllocateUninitializedFixedDoubleArray(int, v8::internal::PretenureFlag) [/usr/local/bin/node]
5: v8::internal::Factory::NewFixedDoubleArray(int, v8::internal::PretenureFlag) [/usr/local/bin/node]
6: v8::internal::(anonymous namespace)::ElementsAccessorBase >::GrowCapacity(v8::internal::Handle, unsigned int) [/usr/local/bin/node]
7: v8::internal::Runtime_GrowArrayElements(int, v8::internal::Object**, v8::internal::Isolate*) [/usr/local/bin/node]
8: 0x1d902ef0427d
```

It fails similarly with json2geobuf, even after modifying the shebang to increase max_old_space_size.

The source file elb.json is here: https://www.dropbox.com/s/0wakul4l5n3zihv/elb.json?dl=1

Reproduce the failure with the linked elb.json input and the shown geobuf.encode call. Start at encode.js:205 in populateLine and trace its use from writeMultiPolygon; done means the defective MultiPolygon input no longer causes invalid array length or heap exhaustion during encoding.

Written by the indexing model from the issue text.