remove redundant condition in memccpy

Commit d9bdfd16 ("fix memccpy to not access buffer past given size")
correctly added a check for 'n' nonzero, but made the pre-existing test
'*s==c' redundant: n!=0 implies *s==c. Remove the unnecessary check.

Reported by Alexey Izbyshev.

src/string/memccpy.c

@@ -29,6 +29,6 @@ void *memccpy(void *restrict dest, const void *restrict src, int c, size_t n)
 #endif
 	for (; n && (*d=*s)!=c; n--, s++, d++);
 tail:
-	if (n && *s==c) return d+1;
+	if (n) return d+1;
 	return 0;
 }