CSS Parser: skip unpaired bracket } & fix NSUInteger underflow

[liuslevis]

When parsing @"s1{p1}}s2{p2}", code :

NSUInteger  braceLevel = 0;
...
braceLevel = MAX(braceLevel - 1, 0) 

will underflow, which lead to braceLevl = NSUIntegerMAX, and rest of cssString will ignore.

Trying to fix this problem.

[coveralls]

Coverage decreased (-0.01%) to 58.846% when pulling 01fb5c1 on SYSUDevTeam:develop into 2e5770e on Cocoanetics:develop.

[odrobnik]

@liuslevis could you Please also add a unit test for the fixed problem?

[liuslevis]

@odrobnik a test case has been added.

[coveralls]

Coverage increased (+0.02%) to 58.879% when pulling eb9bf97 on SYSUDevTeam:develop into 2e5770e on Cocoanetics:develop.

[odrobnik]

Thank you for your contribution!