T bugfix: include size of 'length' to allocated memory

The way allocation is done (granularity of 64KiB) makes a crash more
difficult to achieve, but if a trace is exactly 64KiB, we miss 4 bytes
in the realloc and then anything may happen.

common/utils/T/tracer/record.c

@@ -133,8 +133,8 @@ int main(int n, char **v)
     int vpos = 0;
 
     if (fullread(socket, &length, 4) == -1) goto read_error;
-    if (ebuf.omaxsize < length) {
-      ebuf.omaxsize = (length + 65535) & ~65535;
+    if (ebuf.omaxsize < length + 4) {
+      ebuf.omaxsize = (length + 4 + 65535) & ~65535;
       ebuf.obuf = realloc(ebuf.obuf, ebuf.omaxsize);
       if (ebuf.obuf == NULL) { printf("out of memory\n"); exit(1); }
     }