Fix panic on obtaining provider schemas (hashicorp#1048)

* Add a test case for a sourced schema without a version.

This is now possible since we delay the execution of obtaining
the Terraform version.

* Only check for a matching schema if we have version and schema.

Author: dbanck

internal/state/provider_schema.go

@@ -216,12 +216,14 @@ func (s *ProviderSchemaStore) schemaExists(addr tfaddr.Provider, pCons version.C
 
 	for item := it.Next(); item != nil; item = it.Next() {
 		ps, ok := item.(*ProviderSchema)
-		if ok {
-			if ps.Schema == nil {
-				// Incomplete entry may be a result of provider version being
-				// sourced earlier where schema is yet to be sourced or sourcing failed.
-				continue
-			}
+		if !ok {
+			continue
+		}
+		if ps.Schema == nil || ps.Version == nil {
+			// Incomplete entries may result from the provider version being
+			// sourced earlier where the schema is yet to be sourced,
+			// or vice versa, or sourcing failed.
+			continue
 		}
 
 		if providerAddrEquals(ps.Address, addr) && pCons.Check(ps.Version) {

internal/state/provider_schema_test.go

@@ -877,6 +877,17 @@ func TestAllSchemasExist(t *testing.T) {
 			ExpectedMatch: true,
 			ExpectedErr:   false,
 		},
+		{
+			Name: "missing provider version in schema store",
+			Requirements: map[tfaddr.Provider]version.Constraints{
+				tfaddr.MustParseProviderSource("hashicorp/test"): version.MustConstraints(version.NewConstraint(">=1.0.0")),
+			},
+			InstalledProviders: InstalledProviders{
+				tfaddr.MustParseProviderSource("hashicorp/test"): nil,
+			},
+			ExpectedMatch: false,
+			ExpectedErr:   false,
+		},
 	}
 
 	for _, tc := range testCases {