Check the count in the first word of __CTOR_LIST__ before executing

the loop that invokes the static constructors.  That makes it safe
to link c++rt0.o into any shared library, even one that does not
have any static constructors.  Formerly, doing that would cause a
bus error.  If the library has no static constructors, __CTOR_LIST__
comes out as a simple 4-byte COMMON region, and it does not have
the usual NULL word that terminates the list of constructors.  This
caused the old code to "call" a garbage address via the non-existent
entry __CTOR_LIST__[1].

The analogous code that invokes the static destructors was already safe.

This change is fully backward-compatible.

Reviewed by:	dfr@render.com (Doug Rabson)

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