"fix mixed_vector bug" (#9319)

paddle/fluid/framework/mixed_vector.h

@@ -176,7 +176,7 @@ class Vector {
 
   // resize the vector
   void resize(size_t size) {
-    if (size + 1 < capacity()) {
+    if (size + 1 <= capacity()) {
       size_ = size;
     } else {
       MutableCPU();

paddle/fluid/framework/mixed_vector_test.cu

@@ -104,3 +104,11 @@ TEST(mixed_vector, ForEach) {
   for (auto& v : tmp) {
   }
 }
+
+TEST(mixed_vector, Reserve) {
+  paddle::framework::Vector<int> vec;
+  vec.reserve(1);
+  vec.push_back(0);
+  vec.push_back(0);
+  vec.push_back(0);
+}