Bug 4376: clang cannot build Squid eCAP code

Bug 4376: clang cannot build Squid eCAP code

This change is not needed for libecap v1.0 that uses tr1::shared_ptr.
However, libecap is being updated to use std::shared_ptr. Once that
update is complete, Squid eCAP code will no longer compile because the
implicit conversion of shared_ptr to bool in tr1 was deemed dangerous
and, hence, made "explicit" when shared_ptr became standard. Unlike "if"
statements, "return" statements do not trigger an explicit conversion.