avoid calling memset with a negative count

avoid calling memset with a negative count

Poorly written perl code that allows an attacker to specify the count to
perl's 'x' string repeat operator can already cause a memory exhaustion
denial-of-service attack. A flaw in versions of perl before 5.15.5 can
escalate that into a heap buffer overrun; coupled with versions of glibc
before 2.16, it possibly allows the execution of arbitrary code.

The flaw addressed to this commit has been assigned identifier
CVE-2012-5195.