Check before calling Vec::reserve in spec_extend

Author: saethlin

library/alloc/src/vec/spec_extend.rs

@@ -35,7 +35,9 @@ where
             );
         }
         if let Some(additional) = high {
-            self.reserve(additional);
+            if additional > self.capacity().wrapping_sub(self.len()) {
+                self.reserve(additional);
+            }
             unsafe {
                 let mut ptr = self.as_mut_ptr().add(self.len());
                 let mut local_len = SetLenOnDrop::new(&mut self.len);