Allow PUSHS to be used before a section

Previously, a PUSHS before a SECTION directive would cause rgbasm to crash when encountering a subsequent POPS.

This is because the subsequently-called out_setCurrentSection() expected the new section to be non-null, which wasn’t the case in this situation. This has been addressed by allowing the ‘null’ section to be set in this function, and only dereferencing it (to set nPC) if a non-null section is to be set.

In practice, this means that PUSHS/POPS can now be used to push/restore a context without a section.

src/asm/output.c

@@ -1,7 +1,7 @@
 /*
  * This file is part of RGBDS.
  *
- * Copyright (c) 1997-2018, Carsten Sorensen and RGBDS contributors.
+ * Copyright (c) 1997-2019, Carsten Sorensen and RGBDS contributors.
  *
  * SPDX-License-Identifier: MIT
  */
@@ -655,7 +655,7 @@ void out_SetCurrentSection(struct Section *pSect)
 		fatalerror("Cannot change the section within a UNION");
 
 	pCurrentSection = pSect;
-	nPC = pSect->nPC;
+	nPC = (pSect != NULL) ? pSect->nPC : 0;
 
 	pPCSymbol->nValue = nPC;
 	pPCSymbol->pSection = pCurrentSection;

test/asm/pops-no-pushed-sections.asm

@@ -0,0 +1 @@
+POPS

test/asm/pops-no-pushed-sections.out

@@ -0,0 +1,2 @@
+ERROR: pops-no-pushed-sections.asm(1):
+    No entries in the section stack

test/asm/pops-no-pushed-sections.out.pipe

@@ -0,0 +1,2 @@
+ERROR: -(1):
+    No entries in the section stack

test/asm/pops-restore-no-section.asm

@@ -0,0 +1,10 @@
+PUSHS
+
+SECTION "Test", ROM0
+SomeContent:
+	db 1, 2, 3, 4, 5, 6, 7, 8, 9
+
+POPS
+
+DisallowedContent:
+	db 0

test/asm/pops-restore-no-section.out

@@ -0,0 +1,2 @@
+ERROR: pops-restore-no-section.asm(10):
+    Code generation before SECTION directive

test/asm/pops-restore-no-section.out.pipe

@@ -0,0 +1,2 @@
+ERROR: -(10):
+    Code generation before SECTION directive

test/asm/pushs-outside-section.asm

@@ -0,0 +1,2 @@
+PUSHS
+POPS