Add missing varasm DECL_P check.

Add missing varasm DECL_P check.

This fixes a riscv64-linux bootstrap failure.

get_constant_section calls the select_section target hook, and select_section
calls get_named_section which calls get_section.  So it is possible to have
a constant not a decl in both of these functions.  They already call DECL_P
checks everywhere except for the new code HJ recently added.  This adds the
missing DECL_P check.

	gcc/
	* varasm.c (get_section): Add DECL_P check before DECL_PRESERVE_P.