add_pathsep(): Return false if filename is too long.

References #3042

src/nvim/path.c

@@ -391,19 +391,22 @@ char *concat_fnames_realloc(char *fname1, const char *fname2, bool sep)
                           fname2, len2, sep);
 }
 
-/*
- * Add a path separator to a file name, unless it already ends in a path
- * separator.
- */
-void add_pathsep(char *p)
+/// Adds a path separator to a filename, unless it already ends in one.
+///
+/// @return `true` if the path separator was added or already existed.
+///         `false` if the filename is too long.
+bool add_pathsep(char *p)
   FUNC_ATTR_NONNULL_ALL
 {
   const size_t len = strlen(p);
-  const size_t pathsep_len = sizeof(PATHSEPSTR);
-  assert(len < MAXPATHL - pathsep_len);
   if (*p != NUL && !after_pathsep(p, p + len)) {
+    const size_t pathsep_len = sizeof(PATHSEPSTR);
+    if (len > MAXPATHL - pathsep_len) {
+      return false;
+    }
     memcpy(p + len, PATHSEPSTR, pathsep_len);
   }
+  return true;
 }
 
 /// Get an allocated copy of the full path to a file.