Fix MQTT SUBSCRIBE parsing

Make sure topic is properly NUL-terminated.
Ignore SUBSCRIBE requests with no topic expressions.

PUBLISHED_FROM=a00f39dda44fe63299e971a91a98f8ee57dd2a64

examples/examples.mk

@@ -21,6 +21,11 @@ ifeq ($(SSL_LIB),mbedtls)
 CFLAGS += -DMG_ENABLE_SSL -DMG_SSL_IF=MG_SSL_IF_MBEDTLS -DMG_SSL_MBED_DUMMY_RANDOM -lmbedcrypto -lmbedtls -lmbedx509
 endif
 
+ifdef ASAN
+CC = clang
+CFLAGS += -fsanitize=address
+endif
+
 $(PROG): $(SOURCES)
 	$(CC) $(SOURCES) -o $@ $(CFLAGS)