tree-wide: introduce mfree()

Pretty trivial helper which wraps free() but returns NULL, so we can
simplify this:
        free(foobar);
        foobar = NULL;
to this:
        foobar = mfree(foobar);

src/basic/copy.c

@@ -467,8 +467,7 @@ int copy_xattr(int fdf, int fdt) {
 
                 sza *= 2;
 
-                free(bufa);
-                bufa = NULL;
+                bufa = mfree(bufa);
         }
 
         p = bufa;
@@ -491,8 +490,7 @@ int copy_xattr(int fdf, int fdt) {
                         if (m < 0) {
                                 if (errno == ERANGE) {
                                         szb *= 2;
-                                        free(bufb);
-                                        bufb = NULL;
+                                        bufb = mfree(bufb);
                                         continue;
                                 }
 

src/basic/util.h

@@ -84,6 +84,11 @@ bool streq_ptr(const char *a, const char *b) _pure_;
 
 #define malloc0(n) (calloc((n), 1))
 
+static inline void *mfree(void *memory) {
+        free(memory);
+        return NULL;
+}
+
 static inline const char* yes_no(bool b) {
         return b ? "yes" : "no";
 }