func_curl.c: Prevent crash when using CURLOPT(httpheader)

Because we use shared thread-local cURL instances, we need to ensure
that the state of the cURL instance is correct before each invocation.

In the case of custom headers, we were not resetting cURL's internal
HTTP header pointer which could result in a crash if subsequent
requests do not configure custom headers.

ASTERISK-29085 #close

Change-Id: I8b4ab34038156dfba613030a45f10e932d2e992d