Pacify GCC when compiling unexelf.c on Fedora 30

* src/unexelf.c (unexec): Pacify GCC 9.

src/unexelf.c

@@ -304,6 +304,7 @@ unexec (const char *new_name, const char *old_name)
 	      || seg->p_vaddr > old_bss_seg->p_vaddr))
 	old_bss_seg = seg;
     }
+  eassume (old_bss_seg);
 
   /* Note that old_bss_addr may be lower than the first bss section
      address, since the section may need aligning.  */