pgsql: Fix ERROR message in injection_point.c

Fix ERROR message in injection_point.c

This commit fixes an error message that failed to show the correct
function and library names when a function cannot be loaded.

While on it, adjust the call to load_external_function() so as this
ERROR can be reached, by making load_external_function() return NULL
rather than fail if a function cannot be found for a given injection
point.

Thinkos in d86d20f0ba79.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/cdd863480c27b406ac14e94f1fe122b98ea562b4

Modified Files
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src/backend/utils/misc/injection_point.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)