[fortran,patch] Fix Cray pointers in modules

[FX]

The attached one-line patch fixes PR45187 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=45187), where we would sometimes try to create the backend_decl of Cray-pointees twice. We should simply bail out early of the procedure, following what is done in the similar situation in gfc_finish_var_decl().

Bootstrapped and regtested on x86_64-apple-darwin13, includes a testcase.
OK to commit?

FX

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