Fix for liquid soil moisture < 0 in some cases.

This fixes issue #123.

Tested the following cases:
1. 24h water balance mode, no bare soil: same result as VIC.4.1.2.l
2. 1h energy balance mode, no bare soil: same result as VIC.4.1.2.l
3. 24h water balance mode, bare soil present: fixed water balance errors
4. 1h energy balance mode + frozen soil, bare soil present: fixed water balance errors