swtpm: Improve handling of case where received state has zero length

Skip the malloc and reception of data in case blob_length is 0. Previoulsy
malloc(0) was called when blob_length was 0. malloc returned a valid
pointer with glibc in this case that could be freed. The same is true for
musl libc on Linux or glibc on Cygwin.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>