Fix #9487: GIFLoad array subscript below array bounds

Recently enabled variable range propagation enables GCC to among others analyze
whether array subscript is in a valid range. While being quite useful this
particular feature also happens to produce false positives. This is merely
a workaround to make compiler happy.

The actual reason why compiler is reporting false positive is that array is
indexed with a signed integer and it is not clear for compiler what value it may
have due to it being a member of the class and and external procedure being
invoked between its initialization and usage.