Fix garbage characters at EOF causing an infinite loop

test/asm/garbage-before-eof.asm

@@ -0,0 +1 @@
+MACRO�

test/asm/garbage-before-eof.err

@@ -0,0 +1,5 @@
+error: Invalid character 0xEB (is the file UTF-8?)
+    at garbage-before-eof.asm(1)
+error: syntax error, unexpected end of buffer, expecting ? or symbol
+    at garbage-before-eof.asm(1)
+Assembly aborted with 2 errors!