Backport !1218 “gdb: Fix GHashTable pretty printer off-by-one error”
Commit 7678b107 seems to have left the GHashTable pretty printer with an off-by-one error, skipping the first key it encounters and printing an extra garbage key/value pair instead. This fixes that by moving an increment to the end of a loop rather than the beginning.
Trivial backport of !1218 (merged) to glib-2-62
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