Workaround strncpy() in OpenArena QVMs

Changes in macOS 10.9 SDK and glibc 2.37 broke overlapping dest and src
in strncpy() used by OpenArena QVMs. (Technically it was undefined
behavior.)

In the QVM strncpy() syscall do not alter the dest if dest and src are
equal. This fixes weapon barrel models for OpenArena 0.8.8.

Author: zturtleman

code/client/cl_cgame.c

@@ -618,7 +618,14 @@ intptr_t CL_CgameSystemCalls( intptr_t *args ) {
 		Com_Memcpy( VMA(1), VMA(2), args[3] );
 		return 0;
 	case CG_STRNCPY:
-		strncpy( VMA(1), VMA(2), args[3] );
+		{
+			char *dest = VMA(1);
+			char *src = VMA(2);
+
+			if ( dest != src ) {
+				strncpy( dest, src, args[3] );
+			}
+		}
 		return args[1];
 	case CG_SIN:
 		return FloatAsInt( sin( VMF(1) ) );

code/client/cl_ui.c

@@ -1005,7 +1005,14 @@ intptr_t CL_UISystemCalls( intptr_t *args ) {
 		return 0;
 
 	case UI_STRNCPY:
-		strncpy( VMA(1), VMA(2), args[3] );
+		{
+			char *dest = VMA(1);
+			char *src = VMA(2);
+
+			if ( dest != src ) {
+				strncpy( dest, src, args[3] );
+			}
+		}
 		return args[1];
 
 	case UI_SIN:

code/server/sv_game.c

@@ -809,7 +809,14 @@ intptr_t SV_GameSystemCalls( intptr_t *args ) {
 		return 0;
 
 	case TRAP_STRNCPY:
-		strncpy( VMA(1), VMA(2), args[3] );
+		{
+			char *dest = VMA(1);
+			char *src = VMA(2);
+
+			if ( dest != src ) {
+				strncpy( dest, src, args[3] );
+			}
+		}
 		return args[1];
 
 	case TRAP_SIN: