dsp: bios_table: Null check for pldm_bios_table_iter_is_end()

GCC's -fanalyzer identified the following:

```
In file included from ../tests/dsp/bios_table_iter.c:15:
../src/dsp/bios_table.c: In function ‘pldm_bios_table_iter_is_end’:
../src/dsp/bios_table.c:991:17: error: dereference of NULL ‘iter’ [CWE-476] [-Werror=analyzer-null-dereference]
  991 |         if (iter->table_len - iter->current_pos <= pad_and_check_max) {
      |             ~~~~^~~~~~~~~~~
```

As a safety measure, return true to indicate the end of the iterator if
the iterator is null.

Fixes: 9c76679224cf ("libpldm: Migrate to subproject")
Change-Id: I18eec144120054de33eb351f9a80dee936118126
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/src/dsp/bios_table.c b/src/dsp/bios_table.c
index 34258af..e595c00 100644
--- a/src/dsp/bios_table.c
+++ b/src/dsp/bios_table.c
@@ -988,6 +988,10 @@
 {
 	ssize_t len;
 
+	if (!iter) {
+		return true;
+	}
+
 	if (iter->table_len - iter->current_pos <= pad_and_check_max) {
 		return true;
 	}