suppress uninitialized variable warnings

Starting GCC 7, warnings about uninitialized variables are issued around
them.  Such warnings could be false positives (all versions of clang do
not warn), but adding initializers there could never be bad things.

bignum.c

@@ -3409,7 +3409,7 @@ rb_absint_numwords(VALUE val, size_t word_numbits, size_t *nlz_bits_ret)
     size_t numbytes;
     int nlz_bits_in_msbyte;
     size_t numwords;
-    size_t nlz_bits;
+    size_t nlz_bits = 0;
 
     if (word_numbits == 0)
         return (size_t)-1;

vm_method.c

@@ -894,7 +894,7 @@ rb_method_entry_at(VALUE klass, ID id)
 static inline rb_method_entry_t*
 search_method(VALUE klass, ID id, VALUE *defined_class_ptr)
 {
-    rb_method_entry_t *me;
+    rb_method_entry_t *me = NULL;
 
     RB_DEBUG_COUNTER_INC(mc_search);