conversions.c: Specify that we only want to parse decimal numbers.

Passing 0 as the last argument to strtoimax() or strtoumax() causes
octal and hexadecimal to be accepted which was not originally
intended. So we now force to only accept decimal.

ASTERISK-29950 #close

Change-Id: I93baf0f273441e8280354630a463df263a8c0edd