internal/lsp/cache: fix NPE in fileWasSaved

Reading the docstring, it looks like the sense of the type assertion was
accidentally inverted in this function. Fix it to what I believe was
intended.

Fixes golang/go#37687

Change-Id: Ief44a996f1a262527c2916d6b78b81dd35b2cf9e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/222217
Run-TryBot: Rohan Challa <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Rohan Challa <[email protected]>

Author: findleyr

internal/lsp/cache/snapshot.go

@@ -787,20 +787,20 @@ outer:
 	return result
 }
 
-// fileWasSaved returns true if the FileHandle passed in has been saved.
-// It accomplishes this by checking to see if the original and current FileHandles
-// are both overlays, and if the current FileHandles is saved while the original FileHandle
-// was not saved.
+// fileWasSaved reports whether the FileHandle passed in has been saved. It
+// accomplishes this by checking to see if the original and current FileHandles
+// are both overlays, and if the current FileHandle is saved while the original
+// FileHandle was not saved.
 func fileWasSaved(originalFH, currentFH source.FileHandle) bool {
 	c, ok := currentFH.(*overlay)
-	if ok {
+	if !ok || c == nil {
 		return true
 	}
-	if originalFH == nil {
+	o, ok := originalFH.(*overlay)
+	if !ok || o == nil {
 		return c.saved
 	}
-	o, ok := originalFH.(*overlay)
-	return ok && !o.saved && c.saved
+	return !o.saved && c.saved
 }
 
 // shouldInvalidateMetadata reparses a file's package and import declarations to