Fix a crash with a malformed PE header

Fix a crash with a malformed PE header

Don't try to read the PE export table when no section contains the RVA
for it.

(I have a PE executable [1] packed with UPX, where the export table data
directory entry contains a RVA which doesn't correspond to any section.
Mistakenly trying to debug this with gdb makes it crash.)

[1] https://cygwin.com/setup/setup-2.898.x86_64.exe

gdb/ChangeLog:

2020-01-02  Jon Turney  <jon.turney@dronecode.org.uk>

	* coff-pe-read.c (read_pe_exported_syms): Don't try to read the
	export table if no section contains it's RVA.