Limit nDecPlaces to a sensible value [-20, 20]

Protect against callers using for example rtl_math_StringFormat_F
with rtl_math_DecimalPlaces_Max in worst case..

Change-Id: I9f143df6ae67b22e7732547c0f7a53b498caf2b8
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/104472
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Jenkins

sal/rtl/math.cxx

@@ -433,6 +433,12 @@ void doubleToString(typename T::String ** pResult,
         break;
     }
 
+    // Too large values for nDecPlaces make no sense; it might also be
+    // rtl_math_DecimalPlaces_Max was passed with rtl_math_StringFormat_F or
+    // others, but we don't want to allocate/deallocate 2GB just to fill it
+    // with trailing '0' characters..
+    nDecPlaces = std::max<sal_Int32>(std::min<sal_Int32>(nDecPlaces, 20), -20);
+
     sal_Int32 nDigits = nDecPlaces + 1;
 
     if (eFormat == rtl_math_StringFormat_F)