gopls/internal/cache: add assertions for telemetry crash

Updates golang/go#64547

Change-Id: I35b2477b8f6182bf6774095f18726104227a2fcd
Reviewed-on: https://go-review.googlesource.com/c/tools/+/569935
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>

Author: adonovan

gopls/internal/cache/analysis.go

@@ -1290,6 +1290,15 @@ func (act *action) exec() (interface{}, *actionSummary, error) {
 				if end == token.NoPos {
 					end = start
 				}
+
+				// debugging #64547
+				if start < token.Pos(tokFile.Base()) {
+					bug.Reportf("start < start of file")
+				}
+				if end > token.Pos(tokFile.Base()+tokFile.Size()+1) {
+					bug.Reportf("end > end of file + 1")
+				}
+
 				return p.PosLocation(start, end)
 			}
 		}