debug/pe: handle files with no string table

debug/pe: handle files with no string table

pecoff.doc (https://goo.gl/ayvckk) in section 5.6 says:

Immediately following the COFF symbol table is the COFF string table.
The position of this table is found by taking the symbol table address
in the COFF header, and adding the number of symbols multiplied by
the size of a symbol.

So it is unclear what to do when symbol table address is 0.
Lets assume executable does not have any string table.

Added new test with executable with no symbol table. The

gcc -s testdata\hello.c -o testdata\gcc-386-mingw-no-symbols-exec.

command was used to generate the executable.

Fixes #16084

Change-Id: Ie74137ac64b15daadd28e1f0315f3b62d1bf2059
Reviewed-on: https://go-review.googlesource.com/24200
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>