swap max crashes

### Describe the issue
VPS with several masternodes running on it; the masternodes will randomly crash when the swapfile gets full.

### Can you reliably reproduce the issue?
Not exactly; but it seems to occur when "used" is pegged 100% of swap file size. Without a swap file, i can recall seeing the issue with 3 or 4 masternodes running.

#### If so, please list the steps to reproduce below:
1. run lots of stuff.
2. watch it crash

### Expected behavior
don't crash. Should catch the memory fault (or whatever the fault is) cleanly and stop with an error message; without corrupting the chain.

### Actual behavior
crash

### Machine specs:
- OS: Ubuntu 16.04 w/ $5 Vultr VPS. 3-5 Masternodes without a swap file; 16ish with 4gb swapfile. Sometimes 18ish with 6gb swapfile, sometimes not until 26 or so.