Fixed incorrect a2_Start() error returns

a2_Start() would occasionally return a bogus handle 16, which was actually
supposed to be -A2_OVERFLOW, indicated an API message buffer overflow; the
negation was missing. (The return value is A2_handle - not A2_errors.)

Also added dedicated error code A2_MSGOVERFLOW for this situation.

Closes #338.

include/a2_types.h

@@ -145,6 +145,7 @@ typedef enum {
   A2_DEFERR(LATEMESSAGE,	"API message arrived late to engine context")\
   A2_DEFERR(MANYARGS,		"Too many arguments to VM program")\
   \
+  A2_DEFERR(MSGOVERFLOW,	"API message buffer overflow")\
   A2_DEFERR(BUFOVERFLOW,	"Buffer overflow")\
   A2_DEFERR(BUFUNDERFLOW,	"Buffer underflow")\
   A2_DEFERR(DIVBYZERO,		"Division by zero")\

src/interface.c

@@ -654,7 +654,7 @@ static A2_handle a2_API_Starta(A2_interface *i, A2_handle parent,
 		return am.b.start.voice;
 	if((res = a2_writemsgargs(st->fromapi, &am, argc, argv,
 			offsetof(A2_apimessage, b.start.a))))
-		return res;
+		return -res;
 	return am.b.start.voice;
 }
 

src/internals.h

@@ -920,7 +920,7 @@ static inline A2_event **a2_GetEventQueue(A2_state *st, A2_handle handle)
 	  case A2_TVOICE:
 		return &((A2_voice *)hi->d.data)->events;
 	  default:
-	  	return NULL;
+		return NULL;
 	}
 }
 
@@ -1057,7 +1057,7 @@ static inline A2_errors a2_writemsg(SFIFO *f, A2_apimessage *m, unsigned size)
 				"%d bytes (min: %d)", size, A2_APIREADSIZE);
 #endif
 	if(sfifo_Space(f) < size)
-		return A2_OVERFLOW;
+		return A2_MSGOVERFLOW;
 	m->size = size;
 	m->b.common.argc = 0;
 	if(sfifo_Write(f, m, size) != size)
@@ -1079,7 +1079,7 @@ static inline A2_errors a2_writemsgargs(SFIFO *f, A2_apimessage *m,
 	if(argc > A2_MAXARGS)
 		return A2_MANYARGS;
 	if(sfifo_Space(f) < size)
-		return A2_OVERFLOW;
+		return A2_MSGOVERFLOW;
 	m->size = size;
 	m->b.common.argc = argc;
 	memcpy((char *)m + argoffs, argv, argsize);