Fix a memory leak

[yan12125]

This patch fixes a memory leak in aur.c. pkg->pkgbase is allocated here, but not free'd in aur_pkg_free().

Tested with:

valgrind --leak-check=full ./src/package-query --aur-url "https://aur4.archlinux.org" -Aif "%u" package-query

Before:

==1378== 
==1378== HEAP SUMMARY:
==1378==     in use at exit: 342 bytes in 12 blocks
==1378==   total heap usage: 38,225 allocs, 38,213 frees, 2,841,826 bytes allocated
==1378== 
==1378== 14 bytes in 1 blocks are definitely lost in loss record 4 of 12
==1378==    at 0x4C29F90: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==1378==    by 0x55650B9: strndup (in /usr/lib/libc-2.21.so)
==1378==    by 0x404868: json_string (aur.c:396)
==1378==    by 0x50AD5A8: ??? (in /usr/lib/libyajl.so.2.1.0)
==1378==    by 0x40413B: aur_json_parse (aur.c:492)
==1378==    by 0x40413B: aur_fetch (aur.c:535)
==1378==    by 0x4046E1: aur_request (aur.c:645)
==1378==    by 0x403D0A: main (package-query.c:542)
==1378== 
==1378== LEAK SUMMARY:
==1378==    definitely lost: 14 bytes in 1 blocks
==1378==    indirectly lost: 0 bytes in 0 blocks
==1378==      possibly lost: 0 bytes in 0 blocks
==1378==    still reachable: 328 bytes in 11 blocks
==1378==         suppressed: 0 bytes in 0 blocks

After:

==1630== 
==1630== HEAP SUMMARY:
==1630==     in use at exit: 328 bytes in 11 blocks
==1630==   total heap usage: 38,213 allocs, 38,202 frees, 2,841,538 bytes allocated
==1630== 
==1630== LEAK SUMMARY:
==1630==    definitely lost: 0 bytes in 0 blocks
==1630==    indirectly lost: 0 bytes in 0 blocks
==1630==      possibly lost: 0 bytes in 0 blocks
==1630==    still reachable: 328 bytes in 11 blocks
==1630==         suppressed: 0 bytes in 0 blocks

[Skunnyk]

Thank you. Feel free to seek for other leaks ;)