mpeg4_enc_fuzzer: Fix sizeof() bugs

When we changed inputBuffer from a stack array to a pointer, it
changed what sizeof(inputBuffer) means (from the entire size of
the array, to just the size of a pointer).

Since we intended to know the entire size of the array, we
replace the sizeof() usages accordingly.

Test: TreeHugger
Change-Id: I0a9515a9e0fe63e258056bfdbf19310560e02504