Fix crash in xml.Unmarshal. Occasionally while running a scrape of some xml with many goroutines

I see a crash with the following stack trace:

	SIGSEGV: segmentation violation
	Faulting address: 0x44
	PC=0x44e510

	runtime·mapaccess2+0x18 /home/vish/go/src/pkg/runtime/hashmap.c:790
       	 runtime·mapaccess2(0x0, 0x0)
	xml·*Parser·text+0x922 /home/vish/go/src/pkg/xml/xml.go:765
       	 xml·*Parser·text(0xe44c9870, 0x7fa3, 0xffffffff, 0x7f00, 0x44b5c4, ...)
	xml·*Parser·RawToken+0x174 /home/vish/go/src/pkg/xml/xml.go:405
         xml·*Parser·RawToken(0xe44c9870, 0x7fa3, 0x0, 0x0)
	xml·*Parser·Token+0x625 /home/vish/go/src/pkg/xml/xml.go:205
       	 xml·*Parser·Token(0xe44c9870, 0x7fa3, 0x0, 0x0, 0x0, ...)
	xml·*Parser·Skip+0x27 /home/vish/go/src/pkg/xml/read.go:371
       	 xml·*Parser·Skip(0xe44c9870, 0x7fa3, 0x8, 0x0)
	xml·*Parser·unmarshal+0xae6 /home/vish/go/src/pkg/xml/read.go:328
       	 xml·*Parser·unmarshal(0xe44c9870, 0x7fa3, 0xe8127120, 0x7fa3, 0xdc94bc40, ...)
	xml·Unmarshal+0xda /home/vish/go/src/pkg/xml/read.go:121
       	 xml·Unmarshal(0xe65724b0, 0x7fa3, 0xdc94bc00, 0x7fa3, 0x4bcfc0, ...)
	scraper·DoSearch+0x3d1 /home/vish/my-go-src/google3/experimental/users/vish/go_http/scraper.go:95
       	 scraper·DoSearch(0xe7dc28c0, 0x7fa3, 0x33, 0x3, 0x6d2510, ...)
	main·scrapeQueries+0x202 /home/vish/my-go-src/google3/experimental/users/vish/go_http/scrape.go:251
       	 main·scrapeQueries(0xe7da2600, 0x7fa3, 0x64, 0x1db0, 0xe7e10000, ...)
	goexit /home/vish/go/src/pkg/runtime/proc.c:135
       	 goexit()
	0x7fa3e7da2600 unknown pc

	Clearly xml.Parser is accessing an uninitialized map. Add a check for a nil map before accessing it.

R=r, rsc
https://golang.org/cl/159050

src/pkg/xml/xml.go

@@ -761,7 +761,7 @@ Input:
 				if r, ok := entity[s]; ok {
 					text = string(r);
 					haveText = true;
-				} else {
+				} else if p.Entity != nil {
 					text, haveText = p.Entity[s]
 				}
 			}