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.

util.c

@@ -3319,6 +3319,9 @@ Perl_repeatcpy(register char *to, register const char *from, I32 len, register I
 {
     PERL_ARGS_ASSERT_REPEATCPY;
 
+    if (count < 0)
+	Perl_croak_nocontext("%s",PL_memory_wrap);
+
     if (len == 1)
 	memset(to, *from, count);
     else if (count) {