Fix failing assertion with backslash at EOF in macro arguments (#1634)

`Expansion::advance()` can increase its offset beyond the size,
so I don't think this assumption was valid in the first place;
`BufferedContent::advance()` should be able to as well.

Author: Rangi42

src/asm/lexer.cpp

@@ -509,8 +509,9 @@ void BufferedContent::advance() {
 	if (offset == std::size(buf)) {
 		offset = 0; // Wrap around if necessary
 	}
-	assume(size > 0);
-	size--;
+	if (size > 0) {
+		size--;
+	}
 }
 
 void BufferedContent::refill() {

test/asm/command-line-symbols.asm

@@ -0,0 +1,3 @@
+assert !strcmp("{FOO}", "hello")
+assert !strcmp("{DEFINED}", "1")
+def FOO equ 42

test/asm/command-line-symbols.err

@@ -0,0 +1,3 @@
+error: command-line-symbols.asm(3):
+    'FOO' already defined at <command-line>
+error: Assembly aborted (1 error)!

test/asm/command-line-symbols.flags

@@ -0,0 +1 @@
+-Weverything -DFOO=hello -DDEFINED

test/asm/for.asm

@@ -41,6 +41,9 @@ for {s}, 3, 30, 3
 	print "{d:x} "
 endr
 	println "-> {d:x}"
+for s, 10
+	println "{d:s}"
+endr
 
 for v, 10
 	println "{d:v}"

test/asm/for.err

@@ -4,7 +4,9 @@ error: for.asm(16):
     FOR cannot have a step value of 0
 warning: for.asm(20): [-Wbackwards-for]
     FOR goes backwards from 1 to 2 by -1
-error: for.asm(45) -> for.asm::REPT~4(51):
-    'v' already defined as constant at for.asm(45) -> for.asm::REPT~4(49)
-FATAL: for.asm(45) -> for.asm::REPT~4(51):
+error: for.asm(46):
+    's' already defined as constant at for.asm(39)
+error: for.asm(48) -> for.asm::REPT~4(54):
+    'v' already defined as constant at for.asm(48) -> for.asm::REPT~4(52)
+FATAL: for.asm(48) -> for.asm::REPT~4(54):
     Failed to update FOR symbol value

test/asm/macro-arg-escape-chars.asm

@@ -0,0 +1,5 @@
+MACRO mac
+	DEF s EQUS "\#"
+	println "{#s:s}"
+ENDM
+mac \\\"\t\r\0\n\{\}\,\(\)\w\

test/asm/macro-arg-escape-chars.err

@@ -0,0 +1,5 @@
+error: macro-arg-escape-chars.asm(5):
+    Illegal character escape 'w'
+error: macro-arg-escape-chars.asm(5):
+    Illegal character escape at end of input
+error: Assembly aborted (2 errors)!

test/asm/macro-arg-escape-chars.out

@@ -0,0 +1 @@
+\\\"\t\r\0\n\{},()w\\

test/asm/string-literal-macro-arg.asm

@@ -0,0 +1,11 @@
+MACRO mac
+	println "\1"
+	println \1
+ENDM
+mac "hello \\\"\t\r\0\n\ ; comment
+ \wor\
+ld"
+mac """goodbye
+cruel\	; comment
+\nworld"""
+mac "\

test/asm/string-literal-macro-arg.err

@@ -0,0 +1,9 @@
+error: string-literal-macro-arg.asm(6):
+    Illegal character escape 'w'
+error: string-literal-macro-arg.asm(11):
+    Illegal character escape at end of input
+error: string-literal-macro-arg.asm(11):
+    Unterminated string
+error: string-literal-macro-arg.asm(11) -> string-literal-macro-arg.asm::mac(4):
+    Unterminated string
+error: Assembly aborted (4 errors)!