Fix memory allocation size in RegisterExtensionExplainOption()

Fix memory allocation size in RegisterExtensionExplainOption()

The allocations used for the static array ExplainExtensionOptionArray,
that tracks a set of ExplainExtensionOption, used "char *" instead of
ExplainExtensionOption as the memory size consumed by one element,
underestimating the memory required by half.

The initial allocation of ExplainExtensionNameArray wants to hold 16
elements before being reallocated, and with "char *" it meant that there
was enough space only for 8 ExplainExtensionOption elements, 16 bytes
required for each element.  The backend would crash once one tries to
register a 9th EXPLAIN option.

As far as I can see, the allocation formulas of GetExplainExtensionId()
have been copy-pasted to RegisterExtensionExplainOption(), but the
internal maths of the copy were not adjusted accordingly.

Oversight in c65bc2e1d14a.

Author: Joel Jacobson <joel@compiler.org>
Discussion: https://postgr.es/m/2a4bd2f5-2a2f-409f-8ac7-110dd3fad4fc@app.fastmail.com
Backpatch-through: 18