append_yylval_tzString should always evaluate its argument

Fixes #762

src/asm/lexer.c

@@ -1455,15 +1455,16 @@ static char const *readInterpolation(void)
 }
 
 #define append_yylval_tzString(c) do { \
+	char v = (c); /* Evaluate c exactly once in case it has side effects. */ \
 	if (i < sizeof(yylval.tzString)) \
-		yylval.tzString[i++] = (c); \
+		yylval.tzString[i++] = v; \
 } while (0)
 
 static size_t appendEscapedSubstring(char const *str, size_t i)
 {
 	/* Copy one extra to flag overflow */
 	while (*str) {
-		int c = *str++;
+		char c = *str++;
 
 		/* Escape characters that need escaping */
 		switch (c) {

test/asm/long-string-constant.asm

@@ -0,0 +1,13 @@
+
+println """This paragraph has exactly 257--that's two hundred and
+fifty-seven--characters altogether. That's just enough for its last
+two characters to be truncated by rgbasm's lexer, since it can't
+handle more than 255-character strings. The final two won't print: !?"""
+
+mac: MACRO
+	println "\1" ; x1
+	println "\1\1\1\1\1\1" ; x6
+ENDM
+
+	mac Hello! ; 6x6 = 36
+	mac This sentence spans forty-three characters. ; 43x6 = 258

test/asm/long-string-constant.err

@@ -0,0 +1,4 @@
+warning: long-string-constant.asm(5): [-Wlong-string]
+    String constant too long
+warning: long-string-constant.asm(13) -> long-string-constant.asm::mac(9): [-Wlong-string]
+    String constant too long

test/asm/long-string-constant.out

@@ -0,0 +1,8 @@
+This paragraph has exactly 257--that's two hundred and
+fifty-seven--characters altogether. That's just enough for its last
+two characters to be truncated by rgbasm's lexer, since it can't
+handle more than 255-character strings. The final two won't print: 
+Hello!
+Hello!Hello!Hello!Hello!Hello!Hello!
+This sentence spans forty-three characters.
+This sentence spans forty-three characters.This sentence spans forty-three characters.This sentence spans forty-three characters.This sentence spans forty-three characters.This sentence spans forty-three characters.This sentence spans forty-three characte