a/ulaw: fix multiple buffer overflows (#432)

a/ulaw: fix multiple buffer overflows (#432)

i2ulaw_array() and i2alaw_array() fail to handle ptr [count] = INT_MIN
properly, leading to buffer underflow. INT_MIN is a special value
since - INT_MIN cannot be represented as int.

In this case round - INT_MIN to INT_MAX and proceed as usual.

f2ulaw_array() and f2alaw_array() fail to handle ptr [count] = NaN
properly, leading to null pointer dereference.

In this case, arbitrarily set the buffer value to 0.

This commit fixes #429 (CVE-2018-19661 and CVE-2018-19662) and
fixes #344 (CVE-2017-17456 and CVE-2017-17457).

Change-Id: Iffa8edfd10cd51514374bbe4b236dfc8509e1222