flow: fix parsing known switch statements

flow: fix parsing known switch statements

The problem is that this code was ignoring the ->case_statement pointer
which is the first statement after a case statement.  It can be a block
as in the following example code:

	switch (2) {
	case 2: {
		frob(); frob(); frob();
		break;
	}
	...

We need to parse the first statement (which can be block statement with
other statements inside) and then check for if the __path_is_null().

This code is still not perfect.  If the default statement comes first it
assumes that it is a match.  Also Smatch needs to handle impossible paths
better for switch statements.  I don't think that's handled correctly...

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>