fix(coverity/510275): linematch out of bounds access (#30687)

Problem:
Int pointer cast to unsigned long pointer causes potential memory
corruption.

Solution:
Cast and store value first, then pass the new pointer.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/nvim/linematch.c b/src/nvim/linematch.c
index 8943e6e8a6..39dfb8eeb9 100644
--- a/src/nvim/linematch.c
+++ b/src/nvim/linematch.c
@@ -150,7 +150,9 @@ static int count_n_matched_chars(mmfile_t **sp, const size_t n, bool iwhite)
 mmfile_t fastforward_buf_to_lnum(mmfile_t s, linenr_T lnum)
 {
   for (int i = 0; i < lnum - 1; i++) {
-    s.ptr = strnchr(s.ptr, (size_t *)&s.size, '\n');
+    size_t n = (size_t)s.size;
+    s.ptr = strnchr(s.ptr, &n, '\n');
+    s.size = (int)n;
     if (!s.ptr) {
       break;
     }