libexif: Fix read buffer overflow

Make sure the number of bytes being copied from doesn't exceed the
source buffer size.

Test: testPocBug_148705132
Bug: 148705132
Change-Id: Ib0f8441f2d0d4ed33c324630a9400a8412209da7
(cherry picked from commit 127f882f67b38def9b5424987c32e21064f4d49c)