linux: Fix reading uninitted ins variable

[GiuseppeCesarano]

While building the library, clang rightly reports a small edge case where the ins variable is read without previous initialization :

[67/143] Compiling C object gum/libfrida-gum-1.0.a.p/backend-linux_gumprocess-linux.c.o
../gum/backend-linux/gumprocess-linux.c:3664:7: warning: variable 'insn' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
 3664 |   if (!created_thread)
      |       ^~~~~~~~~~~~~~~
../gum/backend-linux/gumprocess-linux.c:3710:7: note: uninitialized use occurs here
 3710 |   if (insn != NULL)
      |       ^~~~
../gum/backend-linux/gumprocess-linux.c:3664:3: note: remove the 'if' if its condition is always false
 3664 |   if (!created_thread)
      |   ^~~~~~~~~~~~~~~~~~~~
 3665 |     goto beach;
      |     ~~~~~~~~~~
../gum/backend-linux/gumprocess-linux.c:3658:17: note: initialize the variable 'insn' to silence this warning
 3658 |   cs_insn * insn;
      |                 ^
      |                  = NULL