file: uci_parse_package: fix heap use after free

file: uci_parse_package: fix heap use after free

Fixes following issue which is caused by usage of pointer which pointed
to a reallocated address:

 ERROR: AddressSanitizer: heap-use-after-free on address 0x619000000087 at pc 0x000000509aa7 bp 0x7ffd6b9c3c40 sp 0x7ffd6b9c3400
 READ of size 2 at 0x619000000087 thread T0
     #0 0x509aa6 in strdup (test-fuzz+0x509aa6)
     #1 0x7fc36d2a1636 in uci_strdup util.c:60:8
     #2 0x7fc36d29e1ac in uci_alloc_generic list.c:55:13
     #3 0x7fc36d29e241 in uci_alloc_package list.c:253:6
     #4 0x7fc36d2a0ba3 in uci_switch_config file.c:375:18
     #5 0x7fc36d2a09b8 in uci_parse_package file.c:397:2
     #6 0x7fc36d2a09b8 in uci_parse_line file.c:513:6
     #7 0x7fc36d2a09b8 in uci_import file.c:681:4

 0x619000000087 is located 7 bytes inside of 1024-byte region [0x619000000080,0x619000000480)
 freed by thread T0 here:
     #0 0x51daa9 in realloc (test-fuzz+0x51daa9)
     #1 0x7fc36d2a1612 in uci_realloc util.c:49:8

 previously allocated by thread T0 here:
     #0 0x51daa9 in realloc (test-fuzz+0x51daa9)
     #1 0x7fc36d2a1612 in uci_realloc util.c:49:8

Reported-by: Jeremy Galindo <jgalindo@datto.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>