libpldm: Fix is_string_type_valid

PLDM_STR_TYPE_UNKNOWN is not a valid string type and is used in cases
where the string length is 0.

Signed-off-by: Tom Joseph <rushtotom@gmail.com>
Change-Id: I18afe78d4527430055501eefa6794bfd309c461e
diff --git a/libpldm/firmware_update.c b/libpldm/firmware_update.c
index d3008b1..acb95fd 100644
--- a/libpldm/firmware_update.c
+++ b/libpldm/firmware_update.c
@@ -10,6 +10,7 @@
 {

 	switch (string_type) {

 	case PLDM_STR_TYPE_UNKNOWN:

+		return false;

 	case PLDM_STR_TYPE_ASCII:

 	case PLDM_STR_TYPE_UTF_8:

 	case PLDM_STR_TYPE_UTF_16: