stacktrace: Remove dangerous value access

dlip.dli_fname could be NULL.
So add null check before use this value.

Signed-off-by: Bojun Seo <bojun.seo.0@gmail.com>

src/stacktrace.cc

@@ -121,11 +121,14 @@ static void print_backtrace_symbol(int count, void *addr)
 		}
 		offset = static_cast<int>(static_cast<int *>(addr) -
 					  static_cast<int *>(dlip.dli_saddr));
-		pr_out("%s +%#x ", symbol, offset);
+		pr_out("%s +%#x", symbol, offset);
 		free(symbol);
 	}
-	offset = (int)((char *)addr - (char *)(dlip.dli_fbase));
-	pr_out("(%s +%#x)\n", dlip.dli_fname, offset);
+	if (dlip.dli_fname) {
+		offset = (int)((char *)addr - (char *)(dlip.dli_fbase));
+		pr_out(" (%s +%#x)", dlip.dli_fname, offset);
+	}
+	pr_out("\n");
 }
 
 static void print_backtrace_symbol_flamegraph(void *addr, const char *semicolon)