Move overflow checks into SkipCutBuffer

Previously SkipCutBuffer would check its input parameters to ensure
they were sane, however since bogus values might be the result of
overflows, and overflow protection was recently turned on for
libstagefright, the compiler's overflow checks were performed before
SkipCutBuffer's, resulting in abort rather than just ignoring the
bogus values.
Moving the multiplication by framesize into SkipCutBuffer fixes this.

Change-Id: I1ad6744bb045a5212701bbf6ee44eecb5f318210