Set name_mapping to NULL after freeing

To avoid risk of double-free.
    
Fixes #237.
3 jobs for jfkthame-master-patch-01067 in 7 minutes and 1 second (queued for 3 seconds)
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passed #2479112
mingw-build

00:05:41

passed #2479110
shared-build

00:07:01

passed #2479111
static-build

00:05:26