Consistently use the "g_string_free returns a C string pointer" idiom.

Consistently use the "g_string_free returns a C string pointer" idiom.

g_string_free(str, FALSE) frees the GString container but not the
underlying g_malloc()ed string; instead, it returns a pointer to the
g_malloc()ed string.

Fix those places that didn't already get the string pointer from
g_string_free() to do so rather than manually extracting the string
themselves.

And fix one place that didn't even need to use a string - it was just
scanning a C string without even modifying it.

Change-Id: Ibbf4872bf5b9935b9907f539b6edb1013f3053a5
Reviewed-on: https://code.wireshark.org/review/6532
Reviewed-by: Guy Harris <guy@alum.mit.edu>