internal/imports: fix two "nil pointer in interface" bugs

CL 559635 changed newModuleResolver so that it can return
(nil, err). That means it is no longer safe to unconditionally
convert the first result to a Resolver interface, but we
forgot to check in two places, causing a crash that was
reported by telemetry.

This change adds the two checks.

Fixes golang/go#66490

Change-Id: I3f2b84ed792b1eea179fc0d4d5ee9843281506fc
Reviewed-on: https://go-review.googlesource.com/c/tools/+/574136
Reviewed-by: Peter Weinberger <pjw@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/imports/fix.go

@@ -996,8 +996,10 @@ func (e *ProcessEnv) GetResolver() (Resolver, error) {
 		// already know the view type.
 		if len(e.Env["GOMOD"]) == 0 && len(e.Env["GOWORK"]) == 0 {
 			e.resolver = newGopathResolver(e)
+		} else if r, err := newModuleResolver(e, e.ModCache); err != nil {
+			e.resolverErr = err
 		} else {
-			e.resolver, e.resolverErr = newModuleResolver(e, e.ModCache)
+			e.resolver = Resolver(r)
 		}
 	}
 

internal/imports/mod.go

@@ -314,15 +314,19 @@ func (r *ModuleResolver) ClearForNewScan() Resolver {
 // TODO(rfindley): move this to a new env.go, consolidating ProcessEnv methods.
 func (e *ProcessEnv) ClearModuleInfo() {
 	if r, ok := e.resolver.(*ModuleResolver); ok {
-		resolver, resolverErr := newModuleResolver(e, e.ModCache)
-		if resolverErr == nil {
-			<-r.scanSema // acquire (guards caches)
-			resolver.moduleCacheCache = r.moduleCacheCache
-			resolver.otherCache = r.otherCache
-			r.scanSema <- struct{}{} // release
-		}
-		e.resolver = resolver
-		e.resolverErr = resolverErr
+		resolver, err := newModuleResolver(e, e.ModCache)
+		if err != nil {
+			e.resolver = nil
+			e.resolverErr = err
+			return
+		}
+
+		<-r.scanSema // acquire (guards caches)
+		resolver.moduleCacheCache = r.moduleCacheCache
+		resolver.otherCache = r.otherCache
+		r.scanSema <- struct{}{} // release
+
+		e.UpdateResolver(resolver)
 	}
 }