BUG/MINOR: h1-htx: Fix a signess bug with char data type when parsing chunk size

BUG/MINOR: h1-htx: Fix a signess bug with char data type when parsing chunk size

On some platform, a char may be unsigned. Of course, we should not rely on
the signess of a char to be portable. Unfortunatly, since the commit
a835f3cb ("MINOR: h1-htx: Use a correlation table to speed-up small chunks
parsing") we rely on it to test the value retrieved from the hexadecimal
correlation table when the size of a chunk is parsed.

To fix the bug, we now test the result is in the range [0,15] with a bitwise
AND.

This patch should fix the issue #1272. It is 2.5-specific, no backport is
needed except if the commit above is backported.