Don't use leftover string length values.

Don't use leftover string length values.

Before processing an SMB request or response, set the string length
variable to 0, and set a flag indicating whether we *have* a string
length variable to "false".  Set the latter to "true" only if we
explicitly set the string length, and if it's not set when we process a
counted string, report an error.  (That *shouldn't* happen, but *can*
happen in a malformed packet, such as an NT Create AndX request with a
zero word count, meaning "no word parameters" and thus "no string length
word parameter".)