alloc: Resolve control reaching the end of a non-void function

Resolves:

    alloc.c: In function ‘__mctp_alloc’:
    alloc.c:32:1: error: control reaches end of non-void function [-Werror=return-type]
       32 | }
          | ^
    alloc.c: In function ‘__mctp_realloc’:
    alloc.c:49:1: error: control reaches end of non-void function [-Werror=return-type]
       49 | }
          | ^
    cc1: all warnings being treated as errors

Functions terminating in an assert() won't always have the assert()
present to terminate them.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: If720f43561585ea6b2dcdae59ac70b997b79469f
diff --git a/alloc.c b/alloc.c
index 9edfd6c..b6b6761 100644
--- a/alloc.c
+++ b/alloc.c
@@ -29,6 +29,7 @@
 	if (alloc_ops.m_realloc)
 		return alloc_ops.m_realloc(NULL, size);
 	assert(0);
+	return NULL;
 }
 
 void __mctp_free(void *ptr)
@@ -46,6 +47,7 @@
 	if (alloc_ops.m_realloc)
 		return alloc_ops.m_realloc(ptr, size);
 	assert(0);
+	return NULL;
 }
 
 void mctp_set_alloc_ops(void *(*m_alloc)(size_t),