fix >=0 comparison warning
fix >=0 comparison warning

gcc 4.8 complained:
```
/ssd/deal-git/source/fe/mapping_q_generic.cc:2800:11: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
```

sairajat/dealii,danshapero/dealii,EGP-CIG-REU/dealii,kalj/dealii,EGP-CIG-REU/dealii,naliboff/dealii,naliboff/dealii,danshapero/dealii,sairajat/dealii,EGP-CIG-REU/dealii,naliboff/dealii,kalj/dealii,EGP-CIG-REU/dealii,danshapero/dealii,sairajat/dealii,kalj/dealii,kalj/dealii,sairajat/dealii,danshapero/dealii,naliboff/dealii,sairajat/dealii,kalj/dealii,EGP-CIG-REU/dealii,naliboff/dealii,naliboff/dealii,danshapero/dealii,kalj/dealii,sairajat/dealii,EGP-CIG-REU/dealii,naliboff/dealii,sairajat/dealii,EGP-CIG-REU/dealii,danshapero/dealii,danshapero/dealii,kalj/dealii