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.

@@ -4568,7 +4568,8 @@ static NSOperationQueue *sharedQueue = nil;
 	for (NSNumber *bytes in bandwidthUsageTracker) {
 		totalBytes += [bytes unsignedLongValue];
 	}
-	averageBandwidthUsedPerSecond = totalBytes/measurements;		
+	if (measurements > 0)
+		averageBandwidthUsedPerSecond = totalBytes/measurements;
 }
 
 + (unsigned long)averageBandwidthUsedPerSecond