Fix BANK(@) outside sections causing crashes

src/asm/rpn.c

@@ -143,7 +143,10 @@ void rpn_BankSelf(struct Expression *expr)
 {
 	rpn_Init(expr);
 
-	if (pCurrentSection->nBank == -1) {
+	if (!pCurrentSection) {
+		yyerror("PC has no bank outside a section");
+		expr->nVal = 1;
+	} else if (pCurrentSection->nBank == -1) {
 		makeUnknown(expr, "Current section's bank is not known");
 		expr->nRPNPatchSize++;
 		*reserveSpace(expr, 1) = RPN_BANK_SELF;

src/link/patch.c

@@ -282,7 +282,12 @@ static int32_t computeRPNExpr(struct Patch const *patch,
 			break;
 
 		case RPN_BANK_SELF:
+			if (!patch->pcSection) {
+				error("%s: PC has no bank outside a section");
+				value = 1;
+			} else {
 				value = patch->pcSection->bank;
+			}
 			break;
 
 		case RPN_HRAM:

test/asm/assert-nosect-bank.asm

@@ -0,0 +1 @@
+assert BANK(@) == 1

test/asm/assert-nosect-bank.err

@@ -0,0 +1,3 @@
+ERROR: assert-nosect-bank.asm(1):
+    PC has no bank outside a section
+error: Assembly aborted (1 errors)!