Fix potential buffer overrun.

BUG=27840771

Rationale:
Fixed buffer overrun by recomputing available remaining space at each
point. Note, we could use strlcat for exactly this situation, using the
full buffer length, but since buffer length - 1 was used in other places,
got the code closest to the original.

Change-Id: Id38ba34949b7e25ae8acadce1c24e080aebf06ae