coccinelle: automatically switch some uses of memcpy() → mempcpy()

Inspired by #22520, let's add a coccinelle script that converts this
automatically.

coccinelle/mempcpy.cocci

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+@@
+expression x, y, z;
+@@
+- memcpy(x, y, z);
+- x += z;
++ x = mempcpy(x, y, z);
+@@
+expression x, y, z;
+@@
+- memcpy_safe(x, y, z);
+- x += z;
++ x = mempcpy_safe(x, y, z);

src/basic/hexdecoct.c

@@ -683,8 +683,7 @@ static int base64_append_width(
                         s += indent;
                 }
 
-                memcpy(s, x + width * line, act);
+                s = mempcpy(s, x + width * line, act);
-                s += act;
                 *(s++) = line < lines - 1 ? '\n' : '\0';
                 avail -= act;
         }

src/libsystemd/sd-journal/catalog.c

@@ -125,15 +125,12 @@ static char *combine_entries(const char *one, const char *two) {
 
         /* Body from @one */
         n = l1 - (b1 - one);
-        if (n > 0) {
+        if (n > 0)
-                memcpy(p, b1, n);
+                p = mempcpy(p, b1, n);
-                p += n;
-
         /* Body from @two */
-        } else {
+        else {
                 n = l2 - (b2 - two);
-                memcpy(p, b2, n);
+                p = mempcpy(p, b2, n);
-                p += n;
         }
 
         assert(p - dest <= (ptrdiff_t)(l1 + l2));