Issue #370: Fix problem where stream errors would not be detected

It seems the comparison has always been against the wrong enum, so the check would never trigger previously.

The latest llvm issues a warning like this:
ASIDataDecompressor.m:161:34: Comparison of constant 'NSStreamEventErrorOccurred' (8) with expression of type 'NSStreamStatus' (aka 'enum NSStreamStatus') is always false

[patch from github user 'OpenFibers', thanks!]
Fix an analyser warning in the latest llvm

Error is: ASIHTTPRequest.m:4571:44: Division by zero

I think this may be a false warning, or llvm is assuming that "[bandwidthUsageTracker addObject:...];" has failed for some reason, but it does no harm to double check that we're not going to divide by zero.