PVS-Studio fixes

[Roman Tsisyk]

memcached/internal/memcached.c:340 err V695 Range intersections are possible within conditional expressions. Example: if (A < 5) { ... } else if (A < 2) { ... }. Check lines: 338, 340.
memcached/internal/proto_bin.c:122 err V517 The use of 'if (A) {...} else if (A) {...}' pattern was detected. There is a probability of logical error presence. Check lines: 122, 126.
memcached/internal/proto_bin.c:534 err V547 Expression is always true. Probably the '&&' operator should be used here.
memcached/internal/proto_bin.c:1104 warn V547 Expression 'errstr' is always true.