libbacktrace output: use 2 digits for frame number

We have allocated as most as for 256 frames but most likely the backtrace
will have lass than 100 items. And 2 digit output better alignes with
the output if some item has multiple lines, it is offset 2 spaces that
will align with comma after the number, which looks better.

Author: MartinPulec

src/host.cpp

@@ -1336,18 +1336,17 @@ print_stacktrace_glibc()
         char backtrace2_msg[] = "\nBacktrace symbolic:\n";
         write_all(fd, sizeof backtrace2_msg - 1, backtrace2_msg);
         for (int i = 0; i < num_symbols; i++) {
-                char sym_nr[5];
+                enum { NDIGITS = 2 };
+                char sym_nr[] = { 'X', 'X', ':', ' ' };
                 int num_tmp = i;
-                for (int i = 0; i < 3; ++i) {
+                for (int i = 0; i < NDIGITS; ++i) {
                         if (num_tmp == 0 && i != 0) {
-                                sym_nr[2 - i] = ' ';
+                                sym_nr[NDIGITS - 1 - i] = ' ';
                         } else {
-                                sym_nr[2 - i] = '0' + (num_tmp % 10);
+                                sym_nr[NDIGITS - 1 - i] = '0' + (num_tmp % 10);
                                 num_tmp /= 10;
                         }
                 }
-                sym_nr[3] = ':';
-                sym_nr[4] = ' ';
                 write_all(fd, sizeof sym_nr, sym_nr);
                 // printf("%3d: ", i);
                 backtrace_pcinfo(bt, (uintptr_t) addresses[i], libbt_full_callback,