Avoid the call to OPENSSL_malloc with a negative value (then casted to unsigned)

CLA: trivial
Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2021)

crypto/dso/dso_lib.c

@@ -324,6 +324,9 @@ DSO *DSO_dsobyaddr(void *addr, int flags)
     char *filename = NULL;
     int len = DSO_pathbyaddr(addr, NULL, 0);
 
+    if (len < 0)
+        return NULL;
+
     filename = OPENSSL_malloc(len);
     if (filename != NULL
             && DSO_pathbyaddr(addr, filename, len) == len)