res_ael: Fix pattern matching against literal '+'

When generating the regular expression that matches against existing
extensions, we need to escape literal characters that can also be
regular expression metacharacters. This was already being done for '*'
but we need to do the same for '+'.

In passing, remove some unreachable code - strcmp() is already run
immediately when entering extension_matches().

ASTERISK-14939 #close
Reported by: klaus3000

Change-Id: I8d2cccb3479168fba1b0a6704c52198b396468f1