Fixing the compiler error on gcc, error: control reaches end of non-void function [-Werror=return-type]

Test: atest libavb_host_unittest libavb_host_unittest_sha
Bug: 351985950
Change-Id: Ie6a44cb41f52ade3a0083698188d9360944a0a5c

libavb/avb_cmdline.c

diff --git a/libavb/avb_cmdline.c b/libavb/avb_cmdline.c
index 28f948f..b6793c1 100644
--- a/libavb/avb_cmdline.c
+++ b/libavb/avb_cmdline.c
@@ -208,13 +208,23 @@
 }
 
 static const char* cmdline_get_digest_name(AvbDigestType avb_digest_type) {
+  const char* ret = NULL;
   switch (avb_digest_type) {
     case AVB_DIGEST_TYPE_SHA256:
-      return "sha256";
+      ret = "sha256";
+      break;
     case AVB_DIGEST_TYPE_SHA512:
-      return "sha512";
+      ret = "sha512";
+      break;
       /* Do not add a 'default:' case here because of -Wswitch. */
   }
+
+  if (ret == NULL) {
+    avb_error("Unknown AvbDigestType.\n");
+    ret = "unknown";
+  }
+
+  return ret;
 }
 
 AvbSlotVerifyResult avb_append_options(