Prevent integer-overflow in htmlSkipBlankChars() and xmlSkipBlankChars()

* HTMLparser.c:
* parser.c:
- Cap the return value at INT_MAX.
- The commit range that OSS-Fuzz listed for the fix didn't make
  any changes to xmlSkipBlankChars(), so it seems like this
  issue may still exist.

Found by OSS-Fuzz Issue 44803.
6 jobs for ddkilzer/oss-fuzz-44803-integer-overflow-in-xmlSkipBlankChars in 4 minutes and 22 seconds (queued for 1 second)