jsapi-util-string: Fix return of gjs_get_string_id()

Previously, the code checked if name_p was a valid location, which it
always is. It was supposed to check if name_p pointed to a non-null
string.
4 jobs for january-maintenance in 22 minutes and 27 seconds (queued for 1 second)
Status Job ID Name Coverage
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passed #5194
fedora_clang

00:11:51

passed #5195
fedora_cppcheck

00:00:23

passed #5193
fedora_gcc

00:10:12

 
  Failure
skipped #5196
on_failure