tests: prefer dynamically allocated buffers
Help detecting Valgrind OOB reads and other issues.
Conditional jump or move depends on uninitialised value(s)
at 0x5452886: blobmsg_parse (blobmsg.c:203)
by 0x400A8E: test_blobmsg (tests/test-blobmsg-parse.c:66)
by 0x400A8E: main (tests/test-blobmsg-parse.c:82)
Conditional jump or move depends on uninitialised value(s)
at 0x545247F: blobmsg_check_name (blobmsg.c:39)
by 0x545247F: blobmsg_check_attr_len (blobmsg.c:79)
by 0x5452710: blobmsg_parse_array (blobmsg.c:159)
by 0x400AB8: test_blobmsg (tests/test-blobmsg-parse.c:69)
by 0x400AB8: main (tests/test-blobmsg-parse.c:82)
Conditional jump or move depends on uninitialised value(s)
at 0x54524A0: blobmsg_check_name (blobmsg.c:42)
by 0x54524A0: blobmsg_check_attr_len (blobmsg.c:79)
by 0x5452710: blobmsg_parse_array (blobmsg.c:159)
by 0x400AB8: test_blobmsg (tests/test-blobmsg-parse.c:69)
by 0x400AB8: main (tests/test-blobmsg-parse.c:82)
Ref: http://lists.infradead.org/pipermail/openwrt-devel/2020-January/021204.html
Signed-off-by: Petr Štetiar <ynezz@true.cz>