stack trace support: fix check on symbol name presence

stack trace support: fix check on symbol name presence

The output format of the stack trace is supposed to be different
depending on whether symbol names are available in the build.

However, the check only verified the validity of the pointer, not of the
string pointed to (which could be empty).

This commit fixes the check so that the original output:

mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x5e000
 > /lib/libc-2.10.1.so(_IO_file_doallocate+0x8c) [0x68a38]
 > /lib/libc-2.10.1.so(_IO_doallocbuf+0x6c) [0x78574]
 > /lib/libc-2.10.1.so(_IO_file_overflow+0x184) [0x7763c]
 > /lib/libc-2.10.1.so(_IO_file_xsputn+0x88) [0x76aac]
 > /lib/libc-2.10.1.so(_IO_puts+0xc8) [0x6b64c]
 > /bin/busybox(+0x0) [0x62c60]
 > /bin/busybox(+0x0) [0x4940]
 > /bin/busybox(+0x0) [0x499c]
 > /bin/busybox(+0x0) [0x4e08]
 > /lib/libc-2.10.1.so(__libc_init_first+0x30c) [0x1f84c]
 > /lib/libc-2.10.1.so(__libc_start_main+0xd8) [0x1f9f8]

becomes:

mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x5e000
 > /lib/libc-2.10.1.so(_IO_file_doallocate+0x8c) [0x68a38]
 > /lib/libc-2.10.1.so(_IO_doallocbuf+0x6c) [0x78574]
 > /lib/libc-2.10.1.so(_IO_file_overflow+0x184) [0x7763c]
 > /lib/libc-2.10.1.so(_IO_file_xsputn+0x88) [0x76aac]
 > /lib/libc-2.10.1.so(_IO_puts+0xc8) [0x6b64c]
 > /bin/busybox() [0x62c60]
 > /bin/busybox() [0x4940]
 > /bin/busybox() [0x499c]
 > /bin/busybox() [0x4e08]
 > /lib/libc-2.10.1.so(__libc_init_first+0x30c) [0x1f84c]
 > /lib/libc-2.10.1.so(__libc_start_main+0xd8) [0x1f9f8]

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Masatake YAMATO <yamato@redhat.com>