fix: possible memory error in calling macros with arguments (#434)

The pointer to the currently processed preprocessor variable is
invalidated when the list of preprocessor variables is extended.

Close #434.

check/fixes.frm

@@ -2285,3 +2285,15 @@ Print +s;
 assert succeeded?
 assert result("F3") =~ expr("+D")
 *--#] Issue405 :
+*--#[ Issue434 :
+* Memory error with macros with arguments
+* The loop (appending a variable and calling the macro) is repeated enough
+* to hit the problem (Valgrind error).
+#define var(a) "`~a'"
+#do n=1,{12*2^5}
+  #define x
+  #message `var(`n')'
+#enddo
+.end
+assert succeeded?
+*--#] Issue434 :

sources/pre.c

@@ -310,6 +310,7 @@ UBYTE GetChar(int level)
 */
 					int nargs = 1;
 					PREVAR *p;
+					size_t p_offset;
 					*s++ = 0; namebuf[i-2] = 0;
 					if ( StrICmp(namebuf,(UBYTE *)"random_") == 0 ) {
 						UBYTE *ranvalue;
@@ -367,6 +368,7 @@ UBYTE GetChar(int level)
 					while ( *s ) s++;
 					s++;
 					t = p->argnames;
+					p_offset = p - PreVar;
 					for ( j = 0; j < p->nargs; j++ ) {
 						if ( ( nargs == p->nargs-1 ) && ( *t == '?' ) ) {
 							PutPreVar(t,0,0,0);
@@ -376,6 +378,7 @@ UBYTE GetChar(int level)
 							while ( *s ) s++;
 							s++;
 						}
+						p = PreVar + p_offset;
 						while ( *t ) t++;
 						t++;
 					}