gdb_main.c: fix buffer overflow on large reads

When gdb issues a `m xx,200` command, the probe should respond with a
packet of size 2*0x200=1024 which is the size of the packet buffer.

However, the `hexify()` procedures writes 1025 bytes in the buffer.

During my tests, it caused the probe to hang when issuing a `dump`
command. Presumably by overwritting the `cur_target` variable.

src/gdb_main.c

@@ -41,7 +41,7 @@
 #define ERROR_IF_NO_TARGET()	\
 	if(!cur_target) { gdb_putpacketz("EFF"); break; }
 
-static char pbuf[BUF_SIZE];
+static char pbuf[BUF_SIZE+1];
 
 static target *cur_target;
 static target *last_target;