Fix OOB read in DNS resolver

The remote server specifies resplen, the length of the response it
intends to send. anssiz represents the size of the destination buffer.
If the reported resplen is larger than the anssiz, the code correctly
only reads up to anssiz bytes, but returns resplen. so later functions
will access far out of bounds.

The fix ensures that the length of send_vc return does not exceed the
buffer size.

Bug: 161362564
Test: atest pass on HWAddressSanitizer build.
Change-Id: Id4b5df1be4652e4623847b0b0bad0af65b80fdd5