fix(diff): use mmfile_t in linematch

Problem:

Linematch used to use strchr to navigate a string, however strchr does
not supoprt embedded NULs.

Solution:

Use `mmfile_t` instead of `char *` in linematch and introduce `strnchr()`.

Also remove heap allocations from `matching_char_iwhite()`

Fixes: #30505

1 files changed, 14 insertions, 0 deletions

diff --git a/src/nvim/strings.c b/src/nvim/strings.c
index b7a87ae030..118abbae6d 100644
--- a/src/nvim/strings.c
+++ b/src/nvim/strings.c
@@ -496,6 +496,20 @@ char *vim_strchr(const char *const string, const int c)
   }
 }
 
+// Sized version of strchr that can handle embedded NULs.
+// Adjusts n to the new size.
+char *strnchr(const char *p, size_t *n, int c)
+{
+  while (*n > 0) {
+    if (*p == c) {
+      return (char *)p;
+    }
+    p++;
+    (*n)--;
+  }
+  return NULL;
+}
+
 // Sort an array of strings.
 
 static int sort_compare(const void *s1, const void *s2)