Handle tabs after backslash at end of file

Commit 6fbb25c added support for tabs between a \ and the newline it escapes,
but yy_create_buffer() was not updated to handle tabs.

test/asm/line-continuation-whitespace.asm

@@ -0,0 +1,7 @@
+; Test that \ followed by whitespace after a macro invocation at the end of the
+; file doesn't cause a segfault.
+
+bar: MACRO
+ENDM
+
+foo bar baz\