Fix nested undefined interpolation segfault

src/asm/lexer.cpp

@@ -797,11 +797,9 @@ static int peek() {
 	} else if (c == '{' && !lexerState->disableInterpolation) {
 		// If character is an open brace, do symbol interpolation
 		shiftChar();
-
 		if (auto str = readInterpolation(0); str) {
 			beginExpansion(str, *str);
 		}
-
 		return peek();
 	}
 
@@ -1201,9 +1199,9 @@ static std::shared_ptr<std::string> readInterpolation(size_t depth) {
 
 		if (c == '{') { // Nested interpolation
 			shiftChar();
-			auto str = readInterpolation(depth + 1);
-
-			beginExpansion(str, *str);
+			if (auto str = readInterpolation(depth + 1); str) {
+				beginExpansion(str, *str);
+			}
 			continue; // Restart, reading from the new buffer
 		} else if (c == EOF || c == '\r' || c == '\n' || c == '"') {
 			error("Missing }\n");

test/asm/nested-bad-interpolation.asm

@@ -0,0 +1,3 @@
+def p = {{a}}
+def q = "{b}"
+def r = "{{c}}"

test/asm/nested-bad-interpolation.err

@@ -0,0 +1,17 @@
+error: nested-bad-interpolation.asm(1):
+    Interpolated symbol "a" is a reserved keyword; add a '#' prefix to use it as a raw symbol
+error: nested-bad-interpolation.asm(1):
+    Interpolated symbol "" does not exist
+error: nested-bad-interpolation.asm(1):
+    syntax error, unexpected end of line
+error: nested-bad-interpolation.asm(2):
+    Interpolated symbol "b" is a reserved keyword; add a '#' prefix to use it as a raw symbol
+warning: nested-bad-interpolation.asm(2): [-Wobsolete]
+    Treating multi-unit strings as numbers is deprecated
+error: nested-bad-interpolation.asm(3):
+    Interpolated symbol "c" is a reserved keyword; add a '#' prefix to use it as a raw symbol
+error: nested-bad-interpolation.asm(3):
+    Interpolated symbol "" does not exist
+warning: nested-bad-interpolation.asm(3): [-Wobsolete]
+    Treating multi-unit strings as numbers is deprecated
+error: Assembly aborted (6 errors)!