Fix search of large memory area in gdbserver

Fix search of large memory area in gdbserver

If the search area is bigger than SEARCH_CHUNK_SIZE (16000), then you get
an error in gdbserver:
gdb: (gdb) find /w 0x3c43f0,+20000,0x04030201
gdb: Pattern not found.
gdbserver: Unable to access 3997 bytes of target memory at 0x3c8273, halting search.

The return value of any additional gdb_read_memory calls were compared with the
wrong value, this fixes it.

gdbserver/ChangeLog:

2020-04-22  Hannes Domani  <ssbssa@yahoo.de>

	* server.cc (handle_search_memory_1): Fix gdb_read_memory return value
	comparison.