uninit_vec: special case set_len(0)

set_len(0) does not create uninitialized elements. Fixes a false positive with
the following pattern:

    fn copy_slice_into_vec(dst: &mut Vec<u8>, src: &[u8]) {
        dst.reserve(src.len().saturating_sub(dst.len()));
        unsafe {
            dst.set_len(0);
            std::ptr::copy_nonoverlapping(src.as_ptr(), dst.as_mut_ptr(), src.len());
            dst.set_len(src.len());
        }
    }

tests/ui/uninit_vec.rs

@@ -91,4 +91,10 @@ fn main() {
         vec1.set_len(200);
         vec2.set_len(200);
     }
+
+    // set_len(0) should not be detected
+    let mut vec: Vec<u8> = Vec::with_capacity(1000);
+    unsafe {
+        vec.set_len(0);
+    }
 }