debug/pe: do not create symbol table if FileHeader.PointerToSymbolTable is 0

https://github.com/tpn/pdfs/raw/master/Microsoft Portable Executable and Common Object File Format Specification - 1999 (pecoff).doc
says this about PointerToSymbolTable:

File offset of the COFF symbol table or 0 if none is present.

Do as it says.

Fixes #17809.

Change-Id: Ib1ad83532f36a3e56c7e058dc9b2acfbf60c4e72
Reviewed-on: https://go-review.googlesource.com/33170Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/debug/pe/symbol.go

@@ -23,6 +23,9 @@ type COFFSymbol struct {
 }
 
 func readCOFFSymbols(fh *FileHeader, r io.ReadSeeker) ([]COFFSymbol, error) {
+	if fh.PointerToSymbolTable == 0 {
+		return nil, nil
+	}
 	if fh.NumberOfSymbols <= 0 {
 		return nil, nil
 	}