quote_for_pmake: fix counter underrun resulting in segfault

while (nbs--) { ... } ends with nbs == -1. Rather than a minimal fix,
introduce mempset() to make these kinds of errors less likely in the
future.

Fixes: https://bugzilla.nasm.us/show_bug.cgi?id=3392815
Reported-by: <13579and24680@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

include/compiler.h

@@ -252,6 +252,13 @@ static inline void *mempcpy(void *dst, const void *src, size_t n)
 }
 #endif
 
+#ifndef HAVE_MEMPSET
+static inline void *mempset(void *dst, int c, size_t n)
+{
+    return (char *)memset(dst, c, n) + n;
+}
+#endif
+
 /*
  * Hack to support external-linkage inline functions
  */