Do not expand empty strings

Fixes #813

src/asm/lexer.c

@@ -751,6 +751,10 @@ static void beginExpansion(size_t distance, uint8_t skip,
 			   char const *str, size_t size, bool owned,
 			   char const *name)
 {
+	/* Do not expand empty strings */
+	if (!size)
+		return;
+
 	distance += lexerState->expansionOfs; /* Distance argument is relative to read offset! */
 	/* Increase the total length of all parents, and return the topmost one */
 	struct Expansion *parent = NULL;

test/asm/expand-empty-string.asm

@@ -0,0 +1,6 @@
+test: MACRO
+v equs "X"
+X equs "" ; should not be expanded
+\1
+ENDM
+	test v 0

test/asm/expand-empty-string.err

@@ -0,0 +1,3 @@
+ERROR: expand-empty-string.asm(6) -> expand-empty-string.asm::test(4):
+    syntax error, unexpected number
+error: Assembly aborted (1 error)!

test/asm/expand-empty-string.simple.err

@@ -0,0 +1,3 @@
+ERROR: expand-empty-string.asm(6) -> expand-empty-string.asm::test(4):
+    syntax error
+error: Assembly aborted (1 error)!