Fix T96691: Heap corruption in file_browse_exec

Regression in [0], also use pad buffer by 1 instead of 2 which is no
longer needed as the trailing slash is no longer added
after allocating the string.

0682af0d

source/blender/editors/space_buttons/buttons_ops.c

@@ -222,13 +222,13 @@ static int file_browse_exec(bContext *C, wmOperator *op)
       /* Do this first so '//' isn't converted to '//\' on windows. */
       BLI_path_slash_ensure(path);
       if (is_relative) {
-        const int path_len = BLI_strncpy_rlen(path, str, FILE_MAX);
         BLI_path_rel(path, BKE_main_blendfile_path(bmain));
-        str = MEM_reallocN(str, path_len + 2);
-        BLI_strncpy(str, path, FILE_MAX);
+        str_len = strlen(path);
+        str = MEM_reallocN(str, str_len + 1);
+        memcpy(str, path, str_len + 1);
       }
       else {
-        str = MEM_reallocN(str, str_len + 2);
+        str = MEM_reallocN(str, str_len + 1);
       }
     }
     else {