app_voicemail: Fix test_voicemail_notify_endl test.

app_voicemail: Fix test_voicemail_notify_endl test.

The test_voicemail_notify_endl test checks the end-of-line
characters of an email message to confirm that they are consistent.
The test wrongfully assumed that reading from the email message
into a buffer will always result in more than 1 character being
read. This is incorrect. If only 1 character was read the test
would go outside of the buffer and access other memory causing
a crash.

The test now checks to ensure that 2 or more characters are read
in ensuring the test stays within the buffer.

ASTERISK-25874 #close

Change-Id: Ic2c89cea6e90f2c0bc2d8138306ebbffd4f8b710