Fix div by 0 panic is resize function. (#9286)

Signed-off-by: Callum Styan <callumstyan@gmail.com>

tsdb/exemplar.go

@@ -279,7 +279,7 @@ func (ce *CircularExemplarStorage) Resize(l int64) int {
 
 	migrated := 0
 
-	if l > 0 {
+	if l > 0 && len(oldBuffer) > 0 {
 		// Rewind previous next index by count with wrap-around.
 		// This math is essentially looking at nextIndex, where we would write the next exemplar to,
 		// and find the index in the old exemplar buffer that we should start migrating exemplars from.

tsdb/exemplar_test.go

@@ -413,7 +413,7 @@ func TestResize(t *testing.T) {
 			expectedMigrated:  50,
 		},
 		{
-			name:              "Zero",
+			name:              "ShrinkToZero",
 			startSize:         100,
 			newCount:          0,
 			expectedSeries:    []int{},
@@ -436,6 +436,14 @@ func TestResize(t *testing.T) {
 			notExpectedSeries: []int{},
 			expectedMigrated:  0,
 		},
+		{
+			name:              "GrowFromZero",
+			startSize:         0,
+			newCount:          10,
+			expectedSeries:    []int{},
+			notExpectedSeries: []int{},
+			expectedMigrated:  0,
+		},
 	}
 
 	for _, tc := range testCases {