http: clarify header buffer size calculation

The header buffer size calculation can from static analysis seem to
overlow as it performs an addition between two size_t variables and
stores the result in a size_t variable. Overflow is however guarded
against elsewhere since the input to the addition is regulated by
the maximum read buffer size. Clarify this with a comment since the
question was asked.

Reviewed-by: Daniel Stenberg <daniel@haxx.se>

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