LIB: Fix missing return code

A return code is required in the case where the guarding assertion is
optimised out.

Change-Id: I783baed1f88f42ae1551e5f89a79b0a054b45295
Signed-off-by: Jackson Cooper-Driver <jackson.cooper-driver@arm.com>
diff --git a/lib/tfm_vprintf/src/tfm_vprintf.c b/lib/tfm_vprintf/src/tfm_vprintf.c
index 3d4abd9..56365eb 100644
--- a/lib/tfm_vprintf/src/tfm_vprintf.c
+++ b/lib/tfm_vprintf/src/tfm_vprintf.c
@@ -31,6 +31,7 @@
     default:
         /* String must start with LOG_MARKER_* */
         assert(0);
+        return NULL;
     }
 }