dexdump: fix a static analyzer complaint

Clang's static analyzer was complaining that the size in
strncat(buf + bufUsed - 1, param, bufSize - (bufUsed - 1));
was greater than the number of bytes available in the buffer. I don't
see why it thinks that.

If we refactor the code slightly, this warning goes away.

Bug: 27101951
Test: mma. Warning is gone.
Change-Id: Id1619ec6d80e20f0c157dbb43d72ac861b8f8064